I had my navel pierced by a professional piercer, and I can honestly say the piercing didn't really hurt at all. It felt kind of like a little pinch, but it was over and done with so quick I hardly noticed it.
It was trying to do yoga AFTER I got it pierced that was a little uncomfortable
Stretching the skin by doing side bends and back arches -- I don't recommend!
I think navel jewelry is cute. Mine is a 2-jeweled curved ring and it's small and not horribly obvious. If you keep them clean and don't fiddle with them too much, they're pretty easy to heal.