St. Petersburg's downtown is the start of the North Bay Trail and travels north along scenic Tampa Bay past numerous parks which offer free parking for bikers. Here's a view of the beach near Barwood Park where I parked and biked both directions from here.
You'll enjoy the marinas, beaches, palm trees, and waterfront views for several miles. You then see signs as you enter the historic Old Northeast neighborhood, and after continuing a short distance, I lost the trail at the bridge at Shell Isle Blvd. so I pedaled over that bridge and around a bit, then back to where I'd lost the trail. I still couldn't see any directional signs. I later learned the trail continues straight and takes you past Coffee Pot Park and beyond.
The trail runs through numerous waterfront parks, past marinas, museums, fountains, the airport, and gives you access to The Pier.
I then looked for the starting point for the scenic 42-mile Pinellas Trail but never found it. Later I learned it's at Bay Shore Dr. and 1st Ave. S. I went right past there but never saw a sign for it or the trail itself. (The entire area was being converted for an Indy style race through the streets in March.)