They can go anywhere, really - but these can collectively be called Christmas tree ornaments. I have a collection I’ve mostly bought or been given, but making many different fine kinds is certainly doable. (And of course they make excellent gifts.)

See the Instructions section for directions for making the asterisked ones (seek them out as named here)...
  

• Filled glass balls*
    
• Christmas decals on glass balls, flecked with white paint "snow" from a finger-flicked toothbrush

• Glittery glue stars*

• Pine cone bells*

• Stuffed fabric/felt critters*

• Make shapes out of pieces of wire (from a hanger will do) - stars come to mind... Paint the wire (nail polish would work too; maybe add glitter) and hang the shape with ribbon.

• Rosebud orbs*

• Teeny-tiny pine cones glued onto a thin wooden heart shape (and studded with cloves?), red ribbon glued on near the top
    
• Victorian angel cutouts glued onto balls or cardboard... but with pictures of family faces (or critter faces - not necessarily family pets?) glued on top of the angel faces

• Blown egg ornaments*

• Spiral-twisted pipe cleaner (colored!; glitzy, even) "icicle" ornaments, perhaps topped with a different-colored ball - fun!

• Small, thin wooden shapes (hearts?) with wood-burned designs, hung with a ribbon

• Balsa wood cut-outs decorated with folk art-like designs using narrow-tip indelible marking pens
    
• Carved pine hearts (curve the edges of 1" board cutouts with a rasp, then sand and finish with an oil rub; if you sandwich 2 back-to-back, the hanging ribbon would go between the layers)
    
• Wild things ball ornaments:  leopard (dark brown irregular spots outlined in black, on beige-y glass balls), tiger (black stripes on orange-y balls), or zebra (black stripes in a chevron pattern on white balls)

• Make ornaments from cut-out shapes from greeting card pictures - many such pictures contain elements that would stand alone (though you might have to glue a colored sheet of paper to the back). Or use other paper pictures, such as from a field guide to butterflies; or stickers?

• Pinwheels*

• Little glass prism drops
  

• Vintage kitchen implements (so many of which even have red or green handles)!

• Cookie cutters - perhaps especially (if you don't need to use them again) spray-glued all over and sprinkled liberally with fine iridescent glitter.

• Scrabble tiles glued into just-across, just-down, or across-&-down words (e.g., "HO HO" & "HO HO", "LOVE" & "PEACE")

• A few arty beads on a hanger

• Whole walnuts covered in foil (press it in to reveal the nut shell’s interesting fissures; stitch thread or thin cord through the foil corners gathered and pinched together at the top)

• Bundles of short cinnamon sticks tied with ribbon, and maybe a small sprig of greenery

• Small styrofoam balls covered with glued-on seeds/legumes in interesting patterns

• Lavendered styrofoam ornaments*

• Little wrapped candies wired onto the branches

• Glass ball ornaments (old or new) decorated with ribbons and old-fashioned stickers (angels, Santas, flowers, musical instruments, etc.)

• Large painted wooden rings (i.e., shaped like a wreath) with a small figure glued onto the inside, ribbon or metal eyelet hanger above it

• Dried and pressed autumn leaves might be sandwiched between layers of clear contact paper.

• Aluminum pie plates can be cut/folded into shapes to use as Christmas light reflectors

• Small fish-eye mirrors (the kind that are glued onto larger side-view mirrors on automobiles) make mesmerizing centers for simple backings. ...Perhaps the backing could be of stiff posterboard (gold and/or silver? - they also make some other metallic colors) - one or two stardust shapes put together?

• Clothes-pin ballerinas*

• Look for tiny dolls or stuffed critters, or just the faces... Make brown felt gingerbread-man "costumes" for them - i.e., the faces (and ears?) stick out from the head part of the costume. (If you aren’t actually wrapping an entire doll in the costume, just stuff the gingerbread man costume with batting.)
   
• Speaking of cookies, make star-shaped cookies with a straw-punched hole in the dough, and hang them on the tree from a ribbon. (Everyone gets to eat them on Christmas day?)
    
• Or gingerbread doves, decorated with white icing in various designs (good ornaments for windows as well as trees - yours or your friends')

• Little felt mittens decorated with beading designs, hung with a loop of cord at the cuff

• Light bulb snowmen*

• Sleigh bells decorated with fancy tole painting
    
• Spatter painted hefty paper cut into festive shapes and hung from ribbons (a great kids' project; good for garlands, too)

• Glued-together noodle starbursts, spray-painted or un- (all sorts of fun shapes to play with! - gather them up when you find them in the store)... Not to mention that squiggly fusilli noodles would make delightful "icicles"!

• Glass tube icicles... Buy narrow glass tubing (7mm?) from a chemical supply house - cut it into several-inch lengths by scoring with a sharp-edged file and snapping them off. Smooth the edged with emery paper and insert short hair pins (cut them off if needed) into the ends to act as hangers.

• Gum wrapper chain*-style icicles made with tiny strips of metallic paper

• Origami shapes

• Tiny quilt squares (or simply the shapes created by gluing felt bits together) lightly stuffed, hung on a loop
   
• Simple shapes cut from old quilts (hearts, stars?), put back-to-back and blanket stitched around the edges (with a button or charm sewn onto one side?) and hung from a ribbon

• Little wreaths made of a gathered ring of smocked fabric; or smocked fabric wrapped around a styrofoam (or real, blown) egg form

• Twigs (or straw, or pine needles) formed into the shape of a 5- or 6-pointed star (i.e., 5 or 6 lengths lashed together with thread at the crossed points)

• "Paint" small batiked shapes onto fabric (stars, birds, fish, angels as tree toppers?) - when the wax is washed out, sew and stuff them

• Crochet tiny one-color baskets to be hung from the branches - fill with some interesting little doodads

• Make small wreaths by gluing buttons onto a balsa wood backing.

• Big buttons, bow-tied onto tree branches with curling ribbon

• A stack of size-graded green buttons can make a tiny Christmas tree - put a star charm or button on top, make a stump out of small brown buttons (you could even add a large red button at the bottom for a tree skirt).  Then you could try white snowmen with black tophats...

• Tiny stuffed animals (a teddy bear tree?)

• Tiny needlepointed "gift boxes" made from 6 little mesh squares... Once they’re filled in with yarn needlework, sew the edges together with yarn to make a box. Tie a ribbon around the box and put a hanger into the bow on top.

• Make a miniature sewing basket... Folded bits of fabric look like larger yardage; sew tiny buttons onto cards; knitting needles can be made from bits of toothpick with a tiny bead glued onto the end; wrap thin thread into hanks of "embroidery floss"; and maybe you can even find miniature scissors to add to the mix. ...Use your imagination! (These make great gifts for needlewomen.)

• Make quilted ball ornaments... Sew 2 miniature patchwork designs to fit the 2 halves of a styrofoam ball (make the outer edges octagonal). Baste the octagons together around the ball and cover the joining with glued-on or sewn-on braid/ribbon.

• Cut small shapes from excess CDs with a small-gauge scroll saw (slowly does it).
   
• Charming old clock faces, hung from ribbons

• Remember God’s eyes? - two sticks (twigs are practical and handsome) lashed together into a cross, colorful yarn or string (or multi-colored raffia) wound around each of the 4 "posts" in turn from center to outside, so that a flat surface of strings is on one side. A loop at the top, a tassel at the bottom and perhaps on the ends of the "arms"... a God’s eye (of Mexican origination, they say).

• Miniature Christmas stockings - with a small toy and a candy cane peeking out from the tops. (For an elegant version, try designs pressed into velvet... Wet the velvet, place it over a rubber stamp, then iron on top of the stamp. Then cut the stocking shape to sew.)

• "Tuzzy-muzzies" (Victorian lace- and/or decorative foil- trimmed paper cones with potpourri-filled netting glued into the opening)

• Fun ice cream cones can be made out of thin brown cardboard such as that in paper towel, toilet paper, or wrapping paper rolls - for the cones - and wads of colorful tissue paper for the scoops.

• Make surprisingly elegant dangly ornaments out of… wine corks?!? And beads and tassels, yes… Make a hole through the corks lengthwise (use a small-diameter drill bit, or burn the hole with a hot awl); on narrow fabric ribbon drawn through the little loop of a silky tassel of the same color - the two ends doubled into a long darning needle - thread from bottom to top: the tassel, a large decorative bead (silver, perhaps; or a small one and then a large one), the cork, another beautiful bead (or two, big and then small)… and then tie the rest of the ribbon into a hanging loop. Gorgeous, actually; kinda Chinese-looking.

• Found objects! - what’s around? (a small yoyo; a tiny nutmeg grater; spray-painted aluminum keys; jewelry; feathers; pine or fir cones; shells; chandelier drops; Groucho glasses!); how about a "scavenger hunt tree"?
   
• Small brass numbers and/or letters of the alphabet (like those designed to screw onto house siding), hung up by ribbons

• Pencil "icicles" for a teacher’s tree

• Make icicles out of gold/silver paper... Cut the ends into points, roll into narrow tubes and hang each from a sparkly thread tied to a decorative cord.

• How about 5 turret-like shells glued into a star?

• Clip-on bow ties!
    

Then there are the ornament hangers - if you don’t have the little commercially-produced hooks on hand, try...

• Bobby pins (they clip onto small branches nicely)

• Paper clips (standard ones not so attractive - though you can find colored-metal ones - you can bend one loop out to make a very usable hook; or look for the interesting rectangular ones, or round spirals)

• Gold or silver twist ties

• Colored pipe cleaners

• Monofilament, or fishing line, loops

• Ribbon loops

• Soutache braid loops

• Jute twine loops