What time of year and what sort of activities are you thinking about? Sightseeing, easy walks, serious hikes, mountain biking, off roading, horse back exploring, fly fishing? Lots of options in Utah.
Couple general comments:
* Lots of options for stay-overs going up I-15. Depending on what is going on at home (and traffic), I sometimes pack up mid afternoon, have dinner, then hit the road approx. 8:00 pm. Stop at Primm approx 11:00 pm; sleep at Buffalo Bill’s back lot (huge and well lit). I leave in am and get to St George easily by 10:00 am (set up camp and have time to go off-roading that afternoon.
* I don’t mind driving I-15 at night, except for the Virgin River Gorge (just north of Mesquite). I enjoy that gorge in day light, but not at night.
* If you are into mountain biking, ATV’s, jeeps, or off road exploring in any way, Moab is a bucket list destination.
* Speaking of bucket list places; the Green River, coming out of Flaming Gorge Reservoir offers great drift boating (both fishing and sight seeing) – I’m going there for first time in April.
* If you hit the eastern part of the state (Moab, Arches NP, Canyon Land NP), make a loop and drive through Monument Valley. I wouldn’t justify a trip solely for Monument Valley, but it’s a cool place to drive through.
* Sedona (AZ) has gorgeous red rock scenery (lot of walking/hiking trails and easy to moderate off-road trails) . Depending on where you are in Utah, you could take a southerly route home and hit I-40. Sedona is less than an hour from Flagstaff. If you want to rent in Sedona or Moab, look up Barlow Jeep Rentals – they will explain things, give you maps, and point you to trails appropriate for your skill level.
* Don’t feel you need to hit everything in Utah in one trip. You only live 6 – 7 hours from the St George, Hurricane, Zion area. Bryce is another hour or two.