Let's get this out of the way first: It gets hot in St. George. Really, really hot. We're talking melt the cover off your scriptures hot. So plan your trip accordingly. Spring and Fall are generally wonderful, winter is fine, and summer can be OK, but you need to plan ahead. Plan any outdoor activities for early in the morning, or after the sun goes down. Find air-conditioned places to be during the afternoon. Have a lot of water on hand. And sunscreen.
The St. George temple is a must see for any Temple Traveler, specifically because it is the first temple built that is still active today. The pioneers worked tirelessly for five and a half years to complete the temple so they would have a place to perform ordinances, since, although the building of the Salt Lake Temple had began much earlier, it wouldn't be finished for 16 more years.
St. George is famous for being the home of Brigham Young during the winters. His old home can be toured, as well as many other 19th century buildings around the city. It gives a glimpse into what life must have been like for the early settlers to Utah. St. George is also home to a fine art museum, focusing on artists from the southwest. Brigham Young was a big part of this temple. He specified where it should be built, and pushed for a higher tower. It was decided to go with the shorter tower, but, after the original was struck by lightning, it was replaced with a higher one, just as President Young had wanted in the first place.
The southern portion of Utah has breathtaking natural scenery like almost nowhere else on earth. Besides Zion National Park, which is nearby, you can enjoy the natural beauty on a smaller scale, by hiking the Red Cliffs Nature Trail. If the heat gets to be too much, stop in at the Sand Hollow Aquatic Center, providing swimming fun for all ages.
St. George, although small, has plenty of wonderful places to eat. Get breakfast or brunch at The Bear Paw Cafe, then stop for a quick tasty lunch at the Lunch Box. Don Pedro's will provide you with fun and family-friendly Mexican food. If fresh, local seasonally inspired entrees is your thing, check out The Painted Pony for lunch or dinner, and you can't leave town without trying at least one of Croshaw's Gourmet Pies.
Or make a fancy evening of it, by getting an upscale dinner at Xetava, then catching a musical at the world-class Tuacahn Theater. There are plenty of ways to beat the heat and enjoying a wonderful vacation in sunny St. George.