We stayed in St. George for quick access to Zion. The Worldmark is nice, and there are also the Villas at South Gate timeshares, which seem to deposit a decent sized inventory on Trading Places exchange site.
From there, we also like to go to Bryce for 2 days, with one overnight. The Grand Staircase Inn in Cannondale is 10-15 miles from Bryce, and rooms are under $100 (we paid $59 off season), with a small store below them. Rooms are basic, but ours was large and clean. There is an excellent fancy restaurant at a small resort a few miles north of Cannondale called the Stone Hearth Grille. It is in a very nice place on a hill with a view of the surrounding area, seems out of place in this remote area but is highly worthwhile to round out the experience. The last few miles to get there were on a graded gravel road.
It's an easy sidetrip to Kodachrome State Park south of Cannondale 15 minutes or so, all on paved road. Beyond that, you can venture into Grand Staircase Escalante national monument. We took the Cockscomb Road as far as Grosvernor Arch, which was pretty impressive. It's all gravel road though, not always smooth, and not good in wet weather. It was bone dry for us.
Have been to all 5 national parks in Utah. Zion and Bryce are probably my favorites, but it's hard to choose. Zion has some good outlying portions accessed from different roads such as Kolob and a few others on the way to the main entrance from St. George, which are good drives/hiking areas as well. We did quicker breeze throughs at Arches and Canyonlands with a one night stay in a Moab motel in between. You can see quite a bit just on the driving routes in these 2 parks. Antelope Canyon in Arizona is also great, a don't miss, guided trip required. Capitol Reef is the sleeper. Incredible rock formations are visible from the main roads, but it is more remote. If you don't have time, that's probably the one you'd have to miss.
If you can do a week in St. George, you can get a reasonably good experience at Zion and Bryce, and with another 4-5 days have time for short side trips to Antelope and Moab with its nearby parks, staying at motels there. I recommend a hike down among the hoodoos in Bryce too, really adds to the experience of just seeing them from the road. Queen's Garden loop is a good one if you only have time for one.
If you stay in St. George, the nearby Snow Canyon park has a lot of beautiful red and white sandstone rock formations and a long open canyon too, makes for a nice day trip. No shortage of scenic wonders in this entire area.