DH and I go to St Augustine often since it's just about a 2.5 hour drive for us. The shops in the historic downtown district, especially along St. George street are fun. Get tickets for the trolley early on, and it will let you get off and back on all throughout the downtown area.
If you are into American art and history, the Lightner Museum is wonderful. If you want more of a "Florida" experience, we love the Alligator Farm Zoological Park.
We also really enjoy some of the boat rides, but especially the Scenic Cruise ones. They take you out far enough to see lots of dolphins and pelicans. Try and sit in the very front of the boat on the first level, outside. It's the best view IMO.
Lots of places to eat, but our favorites are The Raintree for upscale casual dining, and of course Columbia for cuban food.
Sunset Grill on St Augustine beach is a great place the locals seem to like. And we always go to Carmelo's for pizza because it's great. Don't let the fact that it's in a gas station put you off.
Lots of touristy stuff, like the Fountain of Youth and Ripley's Believe It or Not, but a must see is the little house carved out of a tree trunk. This is a free exhibit that is in the parking lot of Ripley's, and it's great fun to go through.
Whatever you do, you'll have fun. We always do, and we've been there many times.