Good hiking/camping spots in UT and CO?

Hi everyone-

I’m taking a road trip from Los Angeles to Boulder, CO in mid-June. I’d like to do some camping and hiking, and will be spending about 3 days in Utah and another 3 days in Colorado. This would be all car camping — no backpacking. Any recommendations?

The number of parks and campsites in UT and CO is astounding, so any advice is welcome. Specific campsites or hiking trials would be appreciated. I’d like to avoid the crowds when possible, although I realize this might be difficult to do in June.