Lots of folks are mentioning the egg shaker--
let me add that a rhythm-impaired shakerist can cause quite a bit of damage wtih a loud egg!!!

If you aren't sure how your rhythm skills are, you'll probably have to get someone you trust, someone who will tell you the truth--someone who knows rhythm when they hear it!--and then play along with someone or some recording. Ask them to tell you honestly if you stayed in time.

Nothing like trying to lead a sing-along with a rhythm-impaired shaky-egg or bass player "joining in." (Or bones--yikes!)
Or--BEST OF ALL--one of those rthythm-impaired folks who collect entire BAGS of 'novelty' rhythm toys, and then sit there, going, "Zip! Whiz! Clangety-clang! Bonk-bonk-bonk! whip-whip-whip-whip-whip-whip-whip! Ding ding ding!..." throughout the entire event. Aaaaaarrrghhhhhhh!!!!!!!!

Denise:^)
...who knows a few 'rhythm-bag' collectors...