I would slightly disagree with glc about heading to Cedar City after Bryce, although it's personal choice. Now if you are camping lodging costs wouldn't be much of an issue and I would get on to booking the Watchman campground in Zion NP. I would actually head south on US89 after UT12 to Mt Carmel Junction and take UT 9 through the Mt Carmel tunnel. It's a fantastic drive into the park and the chance to stop to take in some rock formations at Checkerboard Mesa.
Here is a list of stops west of Denver that spring to mind and don't take long to enjoy.
I-70. Take US6 detour to Loveland Pass. Great views as you cross the Continental divide at around 12000ft.
Colorado National Monument Rim drive between Grand Junction and Fruita CO. (Not so quick and likely to add 2 hours with stops but a great detour)
Goblin Valley SP between I70 and Hanksville UT24. Great rock formations but again would add a couple of hours. (Cool Gas station in Hanksville called Hollow mountain. Store has been created by blasting a hole in the rock and it may be a good place to fill up)
Capital Reef NP. The scenic road is great if you have time. If not, Gifford Homestead is a neat stop and you could stop at Panorama Point and Goosenecks overlook just to the west of Fruita UT.
When you join UT12 about 20 mins south there is Larb Hollow overlook. Lovely views for a 5 min stop.
Just beyond Tropic UT you will see parking for Mossy cave trail. The cave isn't much tbh but it offers a nice 15 min stroll in a quieter part of Bryce. When you leave Bryce you will eventually go under Red canyon Arch that spans the road, just after there is parking and you can cross the road and walk among the red rocks.
As mentioned, Checkerboard Mesa if you take the Mt Carmel route to Zion.
Death valley quick stops. Zabriske Point, Furnace Creek and the Sand Dunes.
Tioga Pass (CA120) As you enter the road from 395 you will see Tioga gas Mart. If you take the turn and continue up the hill past the station you will come to a parking lot with expansive views of Mono Lake. You could stop and stroll at Tuolumne Meadows and also pull up by Tenaya Lake.
These are just a small sample of options and you won't get time for all. You need to get a few more days !! lol