5 days in St Pete's won't be enough time to even start doing everything!!
Would highly recommend one of the dolphin cruises, especially one on the yacht Destiny. It's half a day sailing, watching the dolphins and then you land on a little key where you go shelling and have picnic lunch for a couple of hours. Marvellous. The company also have a motor launch which takes you on a different dolphin experience where you end up on Egmont Key and go snorkelling. We;ve done both and they were so good for both DH me and DD (7 and 8 at the times).
Busch Gardens also has a sister water park - Adventure Island, if you didn't want to do another theme park. We prefer it to BB and TL.
A trip to Tampa is always worthwhile. If the weather is a little off, a visit to the Florida Aquarium is good. "Duck Tours" go off from St Petersberg Pier (well worth a trip too!) and Tampa. It's one of those amphibious vehicles which takes you on a tour of the area showing all the local landmarks and historical things, very good, and then it goes into the water for a trip around the bay. Excellent value for money this was.
Like I say, 5 days ain't gonna be enough!!
If you're into nosying around, travel up from St Pete's to Clearwater beach along the little keys (Treasure Island, Madeira beach), some nice little spots along there and there's Suncoast Bird Sanctuary to stop in at (if you're into birds!). Clearwater is ok, we much prefer St Pete's.
Eating around St Pete's is a doddle. One place we would recommend is Johnny Leverocks seafood and steaks and also Silas Dents. Just sitting and reading all about St Pete's famous hermit is fascinating, the food great and the servings huge. It's on the water so try and get a water view talbe (same in Leverocks).
A trip over the Sunshine Skyway to Sarasota is a great trip, only 45 mins up the bay. Shelling is the national gulf coast pasttime in Florida - you'll get hooked and you'll be out there with everyone else walking up and down the beach. Go just was the tide has turned and you'll find some amazing shells.
The Holiday Inn is where we've stayed twice while in St Petes, one of our trips was purely there for two weeks, no Orlando

We had a great, relaxing time. The hotel is great - all rooms have a water view and IMHO the inland waterway is far more interesting than the gulf views (for which you pay a premium). You get many dolphins in the inland waterway and the views are gorgeous . . . some of the houses there
If you want any info about the hotel, shout, or about anywhere else around there.