Glen Canyon Dam has tours also.
http://www.nps.gov/glca/planyourvisit/guidedtours.htmThe south side of the Grand Canyon has more areas to see the canyon as one drives along the 'side' road & has a long drive along it, if I remember correctly. They have shuttles one can take to see some selections of it.
One could see a lot of Bryce Canyon National Park in one day & an overnight as well as Zion. We did Zion with a day trip, not taking the TT.
Edit addition:
There was a heat wave in the area when we visited Bryce & Zion, therefore we went to higher elevation to spend some time in the area. We camped at a Forest Service Campground (Dixie Nat'l Forest, I believe) at Panguitch Lake & drove the back country to Zion one day. We got into very high elevations on the route, very cool & it wanders through the upper sections of
Cedar Breaks Nat'l Mon't. We wanted to see Bristlecone pines. It was a detour loop back to US 89 using UT 143 & UT 14. A neat drive.
The oldest trees in Cedar Breaks National Monument are the Bristlecone Pine. One Bristlecone is over 1,600 years old. You can view this tree on the Spectra Point Trail.
I didn't know about this: "Cedar Breaks offers a spectacular camping experience at 10,000 feet elevation. Ranger-led evening programs are offered on weekends in the 26-site campground." Might be neat.
If only doing one & it isn't 100 degree weather, Zion would have a lot to do, even wading/splashing in the river there.
One thing I regret doing was taking the kids on a trail ride at Bryce. But it was hot at the time.
