I lived in Savannah from 1996-2000 and have gone back to visit from time to time. Savannah is really a walking city. There are numerous old homes you can tour, walk through the many grassy squares, or take a trolley ride. Tybee Island is about 10 miles from downtown if you want to get some beach time in while there. The nightlife is good with nightly concerts in the Market area and bars along the River Street area...not to mention you can walk around downtown with drink in hand!!!! There is a minor baseball team in town that plays in a stadium built in the very early 1900's. There are numerous seafood restaurants all around town. If I were suggesting a 3 day trip:
Day1- do the trolley tour and walk around downtown and eat in the downtown area
Day2- take a drive out to Tybee, tour Fort Pulaski on the way, tour the Lighthouse, etc. and then just enjoy the beach. Just take a drive around the area and pickup dinner at one of the seafood restaurants on Thunderbolt Island (on the way to Tybee) or somewhere else away from downtown.
Day3-enjoy a little more of the downtown area.
Other things close by (within 30 minutes), Hilton Head Island, if you are a military person then Fort Stewart is close by, to name a couple of things.
Hope this helps.