If I had the time on the second day, I'd take a longer route. Stopping at St. George for the night and then an awesome route that takes you past the Vermillion Cliffs, Kayenta (a 30 minute stop at the John Wesley Powell Museum is worth it), and Monument Valley.

Otherwise, there are plenty of decent places to spend the night in St. George, Cedar City, or Richland, etc., and the scenery on I-15 isn't too shabby itself.