I've done the drive from Las Vegas to Salt Lake City on I-15. Assuming you have plenty of time and you leave in the morning, there are a few possible stops that might be interesting. You can do a quick detour around Kolob Canyons for some nice scenery. You will have to pay to enter Zion National Park but it's worth it. Also explore the Nebo Loop National Scenic Byway to see the red rock cliffs and spires. If you really want to try an alternate route from Las Vegas, some people might go straight North all the way up Highway 93 to Wendover, then I-80 across to Salt Lake City. But since you're starting from Saint George, that really doesn't make sense. I think you should take the faster route along I-15, but then stop at a bunch of places and do mini-detours to maximize the scenery along the way.