One of the only ornaments seen with kimono is the "obi-dome" 帯留め／帯留(oh-bee doh-meh), a sort of brooch or bead threaded through the obi-jime cord worn over the obi.
Obi-dome are a kind of novelty item and are usually worn with casual kimono up through mid-formal ones, depending on the flashiness or use of jewels. Two of the most notable exceptions to this rule are large, playful ones sometimes seen with young women's furisode and huge, high-end ones worn by maiko (apprentice geisha).
|Bottom photo: Maiko Humihana 富美英、Fumisono 富美苑、 Fumitama 富美珠|
photo credit Onihide
Here are some examples from the Rakuten seller "Sou-bien", ranging from cute to chic.
Obi-dome are a bit whimsical by nature, as you can tell by the photos above, so pretty much anything goes. :) If you'd like to make your own, here's a quick tutorial I put together.
|Click to see full-size.|