Any particular places to see/visit, spots to eat, experiences to try? I'll probably try a local guide just to cover the basics but never hurts to ask. Thanks for the feedback!
Make sure you visit the Old Town. Just take a stroll through it and enjoy yourself. Find a small wine bar and sample local red wines (some of the best in the world). Make sure you have at least one meal at any of the khinkali houses (khinkali is a local variation of a soup dumpling; lamb is the filling you want to go for most of all). Take a sulphur bath to relax. Visit Mtatsminda Park to get a great view of the entire city.
Are you confined to Tbilisi only while in Georgia? Because, if not, you must visit Vardzia. It's one of the most breathtaking sights I've ever seen anywhere.
P.S. Oh, and watch out for "chacha" (local spirit that can be as strong as 60% ABV). That will put you on your ass quicker than you think! Georgians are very serious when it comes to drinking rituals. If someone offers you a shot of chacha it is considered an insult to turn it down. The peer pressure applied to one to keep drinking is immense. Don't say I didn't warn you.