We rather like this mini Christmas tree ornament, created by TopperDEL over at Thingiverse. It's maybe a bit meta to hang trees on your tree, which is probably why we like it. Plus it's just really cute.
Alternatively, check out this geometric bauble, designed by cunicode. It's not exactly traditional, but it's gorgeous nonetheless.
Or. if you want to get really really intricate, how about this Santa bauble or this Rudolph ornament, both created by imaterialise.