I believe ABC Cyclery in St Pete rents MTB’s. Little way out from Alafia but Im not sure what is closer. I’ll be in the area the week after Easter and plan to ride at Alafia (not sure what day exactly) if any one wants to go….
If you are going to be in the Miami area, your best option is Oleta River State Park in North Miami Beach. They have more than ten miles of singletrack well worth your trip. They have bike rentals right next to the trail entrance, hardtails and full suspension (which I highly recommend for Oleta trails). Check out http://www.bluemoonmiami.com for info, rates and trail map.
Oleta has everything, from beginner/novice trails to intermediate/advanced trails. Oleta has a lot of twisty, rooty single track, with lots of switchbacks and some sections with loose rocks. If you want something more technical and difficult, you have to go to Markham Park in Sunrise. I’ve heard that there is bike shop close to Markham that will let you try out one of their Giant FS demo bikes at the park. Markham has about 8 miles of the best trails in South Florida.
I would recommend Carter(Loyce Harpe Park), Alafia & Boyette. All three are relatively close to each other and are awesome with plenty of red trails to enjoy. The only problem is where to rent a bike?? All are located in central Florida and are no more than 15 miles apart. I think AJ’s in Brandon rents bikes and they’re not far from Alafia & Boyette. I think these three will give you the most bang for your buck without having to travel too far to get to each different trail. 😎