Hiya Juju, I grew up in St. Pete, north of Sarasota. There is great riding to be had in that area. As a kid, we would load up all of the bikes (6 of them) and go to Sarasota all day riding. My Mom never had a bike growing up and didn't know how to ride. But she wanted to ride with us, so my Dad bought a tandem for the both of them. Very happy childhood memories of biking in Sarasota. It is overall, a very bike friendly place. There are bike lanes and trails all though the area. I don't know if you are taking road bikes or trail bikes but this should help you either way.
Here's a link to a pdf of all kinds of state trails. http://www.visitflorida.com/images/biketrails_b_v5.pdf
You've also got South Lido Key Beach area, down at the tip. There are some nice nature trails there. Very pretty.
Here's a pdf for Myakka River Park. It's like $5 for a car to get in but it's beautiful there. http://www.floridastateparks.org/mya...s/brochure.pdf
You might also want to look up Shamrock Park, if I remember that correctly.
Have fun!!