Savannah is gorgeous. If you are there, you should really look into going to The Breakfast Club on Tybee Island at least once. It is a great local breakfast spot a block from the beach with amazing food. The only catch is you have to stand outside in line to get in.
If you like seafood, Savannah is a great place to stop. The city is known for its beautiful squares. If you are anything like me you ask... who cares about a square (its a park)? Well, if you are anything like me you will realize how beautiful they are when you see them and you will enjoy walking or biking around the city and enjoying all the beautiful squares. There is a TON of history there too. Lots of forts, museums and historical houses/sites.
I might also suggest you stop in Charleston too. Its about an hour and a half further north and is another exceptionally gorgeous city. In fact, I prefer Charleston to Savannah. Great food, beautiful port city with lots of history.
Both cities have neat little "tourist trap" areas with little shops. If you have never been or haven't been in a while, they are fun to walk through... just don't limit your trip to those areas. Both cities are amazing.
Someone mentioned St Augustine too. We stopped there years ago and I found it very "tourist trap" esque. There were some really cool things to see though... they have an AWESOME pirates museum, some great Spanish style architecture and some cool forts. I didn't properly plan the trip though, it was kind of a last minute decision to stop for a couple of days. I'm sure if I planned better I would have enjoyed it much more. I'll go back one of these days.
Sounds like a great trip, enjoy!