The Cathedral is really lovely, as are many of the other historical religious buildings around the city, like the First African Baptist Church, the Congregation Mickve Israel, St. John's Church and so on.
One of the most special things about Savannah is the numerous gorgeous parks there. It's easy to lose track of how much you've walked as you stop by each park-- and well worth it! Forsyth Park is the crown jewel of Savannah's many parks, but they're all absolutely lovely.