Being one of the country's oldest cities, Savannah if full of history. There are tons of tours in Savannah for every agenda. The trick is finding the right one for your taste, or more importantly, the best guide. Don't be afraid to ask around for advice as to which guide is the best. It could make or break the tour.
If you are into history, I would suggest visiting the Bonaventure Cemetery, Wormsloe Plantation, and Fort Pulaski. All are out of downtown, (and not within walking distance of each other, yet there may be tours that take you to them, not sure about that), but all are great places to wander, explore, take photos, etc. Take bugspray and/or sunscreen depending on the time of year.
Also, if you are into Geocaching, Savannah has a lot to offer.