Qualifications: Live in Utah, been camping since I was a zygote, growing up we spent anywhere from 14-45 nights camping each year.
Grand Canyon isn't bad by any means, but if I had to choose where to allot my time between that and Zion's, I'd go with Zion's every time. (not sure if you've been to either already). Grand Canyon either needs more time or just a couple hours to see. You can see so much of it just by pulling into a couple scenic pull-offs and visitor center type places.
With Zion's, once you're in the park there's a bunch of different hikes you can do. Angel's Landing is one of my favorites because the view up there is awesome, and in my opinion even better than looking into the Grand Canyon because at Zion's you kind of "earn" the view. That hike alone takes several hours and that's not including the time you'll want to spend at the top (it sort of plateaus and there's a little bit of exploring/angles/sights to see). Not to mention there's a river running through it you can cool off in.
Canyonlands, Goblin Valley, Bryce Canyon, Moab/Arches are all roughly in between Zions and Colorado, and I would recommend any of them as well. Let me know if you have any questions about these bits, or anything else in Utah since I've probably stayed there before.
Also, this might sound condescending, but you're probably at that age where sunscreen is still super lame and getting tan is super duper cool cuz the babes love it! (speaking from experience) If you won't carry around an umbrella
http://francistapon.com/Travels/Advice/ ... n-Umbrellathen at least get a wide brim hat and a ton of sunscreen that you'll reapply throughout the day. Skin cancer ain't nuthin ta fuck with! (My Dad just had a dime size hole gouged out of his nose to remove skin cancer and suddenly I'm hyper aware).
Also, options are limited coming back I assume, but everyone says Four Corners is super duper lame.
Re: 5 people in a jeep. Is it just a 5-seater jeep standard? Sounds like hell for that kind of distance. Suburban (or similar sized vehicle) with that kind of number is great, 2 in front seats, 2 in middle, 1 person can nap in the back and everyone still sprawls out.