Your transportation needs will really be dictated by what you want to do and where you are staying.  If you are staying downtown, most everything is readily accessible by foot or the Downtown Looper Trolley.  Use The Pinellas Suncoast Transit Authority to get you to most places beyond the range of the Trolley and they have a special trolley service just for the beaches of the barrier island running all the way from Clearwater to St. Pete Beach. If you plan to do a lot of different activities spread out over the area - Busch Gardens up in Tampa and Fort DeSoto south of St. Pete, you may want to invest in a rental car.  Parking is generally easy to find and is relatively inexpensive. Public parking along the beaches is generally never more than $10 for the day and the public parking garages in downtown St. Petersburg run about $5 for the day.  There is also plenty of street parking downtown and the meters are not read on the weekends. Visit Parkopedia.com for detailed parking information in downtown St Pete or download one of their parking apps

Driving can get congested around Baywalk on the weekends but there are two good public parking garages on the adjacent blocks so it is an inconvenience easily lived with because it is so well lit and central to everything. This Downtown Interactive Map can be a great help in getting around.