The Castillo de San Marcos is a must.
If you are driving you can go down to the sister fort about 30 minutes south of St. Aug called Fort Matanzas - it is free and involves a free boat ride across the river to the island where the fort is located. Matanzas also has elevated boardwalk nature trails and is right by the beach. Marineland is a few minutes farther south and another minute or two from there is Washington Oaks State Park, which is very nice and cheap. Both forts frequently have Spanish or British reenactors on the weekends.
St. George street is the main shopping district in the Old Towne, but don't forget the streets on either side (Hipolyta, Spanish, Cordova, etc.) there are little shops and such on all of them. On St. George street is the Spanish Quarter living history museum.
Flagler College does tours and is fabulous architecture and history of Henry Flagler. Across the street from there is the Lightner Museum.
There are more than a few nice churches, especially the Presbyterian church where Flagler attended and helped to fund - amazing architecture inside and out and they are usually open and free.
The Alligator Farm is a nice zoological park - more than just alligators. There are always coupons available around town.
The Lighthouse is near the Alligator farm and you can get into it fairly reasonably (again, look for coupons for anything in St. Augustine) and climb to the top for a nice view. Another block down is Anastasia state park, which is beautiful.
There are sometimes events at the Ampitheater, which is right there as well.
Further down you can go to "St. Augustine Beach" proper for some time on the sand in the sun and surf. Enjoy a Rita's Italian Ice (I'm partial to the Gelati's myself).
Again if you are driving you can head up to Jacksonville for a day - lots of things to do: wonderful zoo, museums (MOSH has lots of interactive kids exhibits), Ft. Caroline (which helped prompt the settlement of St. Augustine by the Spanish - and the French from Caroline then tried to attack St. Aug and vice versa - they are two pieces of the same puzzle), Kingsley Plantation, Big and Little Talbot islands, malls and shopping areas.
Don't waste time or money eating at the Santa Maria in St. Aug over the water - huge waste. Biggest ripoff in the city! There are sooooooooo many better places to eat that are also cheaper! Don't do it!
