We loved our trip to Egmont Key while we were staying at St Pete Beach. We booked a trip - half a day - dolphin watching (and did we see loads of them

), and then you get taken to Egmont Key - what a beautiful place. You can explore the island and then we went snorkelling around these old ruins, like Steve P says.
Highly recommend it
We stayed at the Holiday Inn St Pete Beach, but the boat we got from Tierre Verde marina, just outside St Pete Beach. I think there's a few trips.
Another one (with the same company) we took the previous year was a half day yacht trip watching dolphins, coupled with 2 hour picnic stay on Shell Island - again from St Pete's. You could snorkel from here, as well and that was great. Only thing if it's a hot day there's not much shade . . . but then we spent most of our two hours ashore shelling in the water
It's no more than 2 hours max from Orlando, easy trip and well worth it. There are dozens of hotels on St Pete's beach, B&Bs to 2,3,4, & 5* hotels.
Shout if you need any more info - I'll look up name of boat co if you need
