That's what I've loved about my SW
tank. It's been up over a year and I can still find things I didn't know I had! And it's only a 55.
Regarding the "jelly fish" Most jelly fish are "bad for you" anywhere from a bad sting to sudden death. However, it's highly unlikely you have the sudden death variety in your aquarium as they're rather large. Actual jelly fish will always be suspended in the water column, I don't think there are any that will "rest" or settle on the substrate/rock. If what you saw was on a rock or the glass, i doubt it is actually a jelly fish.
Try taking a snapshot of it so we can help you identify it. (and you might want to post the pic in our Identification Forum) just post a link to that thread in here when you do!