Great topic, Kim, and here's what my poking around has found:
First, Savannah is low country - it's flat. You won't find any challenging hills or dales, but you can probably make up for that with more mileage and areas to cover.
Gorp recommends a couple bike trips, to Skidaway Island and McQueen's Island. They also have a link for a historic Savannah bike tour that doesn't work anymore.
Tybee Island offers plenty of outdoor activities, including hiking and beach biking, and there is some interest about
going to Tybee Island on Saturday, April 5. From what I understand, it's about a 20 minute ride by car (possibly bike-able?), and several ST members interested in going have already announced that they could offer to share their transportation. Here is a bike rental on Tybee Island, the only one I could find: Pack Rat Bicycle Rental & Repair 1403 Butler Ave., Tybee Island 912.786.4013.
Trails.com has
this to say about Skidaway Island State Park trails. I'm not a member, but if you are, I believe they can give you detailed maps and trail ideas.
Fodor's has a no-hills recommended bike workout that may be worth considering. And
Frommer's mentions the fact that many hotels and inns will offer their clients bike rentals. And
here in the Yellow Book, I found a few bike rental places.
Kim, I think you've come up with a great activity while in Savannah. I hope some of this was helpful.