Coming up from Mesquite there is Zions National park which is very nice. Its warm now but a very nice place to see. Its off I-15 a little ways. Then Salt Lake City is nice, go up the canyon to Park City and thats nice to. As for I-15 North to Idaho there is Pocatello, Blackfoot, Idaho falls. Not really a whole to stop and see. When you get to Idaho Falls you can deviate and go to Yellowstone National Park thats always fun. Sorry, not much help. Anyway whatever you decide to do have fun and be safe.