fun for the kids and everyone... plus in the spring, the rock will be weeping very nicely. Also, the emerald pools there are very nice, and a relatively easy hike.
Also, check out Snow Canyon.... 8 miles north of St. George... lots of hikes, lava tubes, sand dunes, red rocks to climb, you can't go wrong.
Biking, St. George has SOOO many biking trails, both paved and dirt.