Hello and welcome to the RTA forums !

The first thing that puzzles me is how you plan to get 5 people and all their luggage and camping equipment into a Sedan? You also mention the tent as a 'reserve' for sleep and the car for sleeping in ? One or two people are not going to get a good nights rest in a sedan never mind more, you really do need the right equipment so that all five of you get a good nights rest or things could get tense real quick.

There are plenty of great spots along the route, others that spring to mind are Bryce canyon and Capitol Reef that would include a drive along Utah scenic 12. Arches and Canyonlands near Moab, the Colorado NM just off I70 and another worthwhile detour that doesn't take a lot of time away, is a detour over the continental divide at Loveland Pass. Take US6 just past Dillon. Then of course there is Rocky mountain NP. On the way back you could take US285 to US50 and head west over Monarch Pass, through Currecanti Nat rec area to Black canyon. Then take the spectacular 'Million dollar Highway' through Ouray and Silverton to Durango. [US550] You would then pass Mesa Verde NP on route to the South rim of the Grand canyon, but not before taking a detour onto 163 through Monument valley. You will have to pick and choose and if you can manage 10 days, use everyone of them.