St. George is a rather interesting small city. If you have any interest in Mormon Pioneer history, the LDS Temple has a Visitor's Center that is always interesting. Since the Mormons were the first white settlers in the area, even if you set aside the religious overtone, the general history of the area is pretty interesting.
Beyond that, St. George has a pretty active shopping area, friendly people, and nice neighborhoods. There are several excellent golf courses in the area, if you happen to be a golfer. There are amazing dinosaur tracks that can be seen pretty easily. I don't recall any restaurants that are amazing, but all the alces I've eaten there were good to better than average. Here's a link to the Chamber of Commerce website "Things to Do" page:
http://www.stgeorgechamber.com/Places to See and Things To Do.htm
And if it's all too much (or too little) for you, it's an easy drive down the freeway to Mesquite, Nevada, if you want to gamble.
If you have three days, be sure to at least drive to Zion National Park. It's less than an hour away, is a very nice, scenic drive, and it's a beautiful park. You can spend a long day and see a lot of the park.
Have fun!
Dave