I think I have done all of those.

For the snowmen and reindeer.. unless your kids are really little please let them do the embellishments, even if you have to guide them. Those crazy lines they draw will mean more than the straight ones adults do. Always remember to date it too.
3-4 year olds always enjoy stringing beads on pipecleaners for some reason. This is cheaper and easier than buttons.
We have used decoupage glue and glitter pens to put their pictures on a ceramic ornament.
Sculpey is big here.. my kids have made some amazing things with that as they got older.
Paint a small terra cotta pot and string it so that there is a hook on one end and a bell inside.
Glass ornaments can be stuffed with all kinds of things. One year we popped popcorn inside of them and decorated the outside. You can do an "I spy" with tiny objects, melted snowman pieces, beach sand, etc."
One of my best parenting hints.. I have put my child's initials and date on the back of every ornament they have received over the years. I only have two, and they are boy/girl, but we still get confused every year about which kid some of them belong to. Then we just flip it over to look.
I give my kids an ornament every year and often I can't find what I am looking for so I make it. I think my favorite is a Harry Potter one I made for DD. I looked up lots of book quotes and typed them up very small. Then I stuffed them into the glass ball. I gave her a list of what they said. (You can read some of them from the outside.) Have fun!