Most of the ones I can think of have already been posted, but here are a few.
If you cross your eyes or stick out your tongue, you'll get stuck that way.
You're not supposed to take a bath during your period because it'll make it stop. (??) However, taking a shower is okay. (Heck, I would've sat in the bathtub all day if it really worked! ROFL!)
If you hear a dove, it's going to rain soon.
If you hear an owl, a loved one is going to pass away soon.
If you find a penny, pick it up. It's supposed to be lucky. There was a second part that said if "heads" was showing it meant one thing, and if "tails" was showing, it meant something else, but I don't remember the rest of it.
If you swallow a watermelon seed, a watermelon will grow in your stomach!
If you eat carrots, you'll be able to see in the dark.