Take 54th Avenue South out of St. Petersburg, and you'll soon hit the Bayway Isles, a quadrangle of islands that are entirely taken up by condos, resorts and houses. Two of the individual islands don't seem to have official names of their own (name them, dammit!) but the northernmost one is Point Brittany Island and the westernmost one is Isla Del Sol. There's also a sandbar island called Cat Point Bank that sometimes exists and sometimes doesn't, depending on the whims of the water level and the shifting sands.
None of these offer a visitor a whole lot to do unless you're staying there or are a yacht club member. However, it is here on Isla Del Sol where you make a left turn to go deeper into the Bayway Islands - continuing straight will take you over to Long Key, Pass-A-Grille, Vina Del Mar Island, Mud Key, and Don CeSar.
Next up are Bird Key (there are way too many Florida islands bearing this same name, unfortunately), Tierra Verde, Pine Key and Paradise Key, which again are completely built up to the extent that it must be great fun to live here, but not terribly exciting to be passing through. (Some maps confuse Paradise Key with Pardee Key, an unpopulated little spot just to the south.)
But hang on, because the area gets a lot more rockin' from here. Next is Cabbage Key (not to be confused with Florida's other Cabbage Key) which lo and behold, actually has a nightlife with restaurants, liquor stores, and other necessary staples of civilization.
If you have a boat, other potentially useful islands in the vicinity include Indian Key, Tarpon Key, Whale Island, Jackass Key, Sawyer Key, Summer Resort Key (which, despite being so named, has nothing on it), Shell Key (a huge bracket shaped island with fantastic beaches and superb shells), Mule Key, Listen Key, and my personal favorite, God's Island.
Meanwhile, back on the causeway, Madelaine Key has a huge public boating/fishing area on its eastern side, with ample parking (five bucks to park though, and that's on top of the tolls you paid to get out here!) and many piers and docks. Next is St. Joan Key, which is almost completely undeveloped and really just serves as a vessel to carry the highway across to Mullet Key. But it also has, I might add, some nice locations with which one can obtain privacy, if in fact privacy on an island is what you seek. St. Christopher Key is accessible by road from St. Joan Key to the west, and offers a campground and a lovely view of the little-seen waterways inside the V-shaped Mullet Key.
Mullet Key is the location of Fort DeSoto Park and has the best beaches of all these keys, if you ask me. Except for the next island - Egmont Key, accessible only by boat, it has a similar "you're at the end of the line" feel as Key West; consider the Bayway Islands your training wheels for the lower keys.
(There's yet one more island here reachable by road - Bonne Fortune Key, sometimes spelled "Bonnie Fortune Key". Alas, when I went the other evening, there's a sign up saying the entire island is down for maintenance. Seriously. I hope they get it fixed soon.)