IMO the best hiking is at Bryce. It is higher altitude and much cooler. Hiking back out of the canyon is rather tiring but worth it. The trails are well graded and excellent.
Arches will be hotter than Hades, get up very early and get your hiking in early before 10 or 11 AM. Carry lots of water. Zion has good hiking but the altitude changes are even more extreme than Bryce and it is hot too.
Capitol Reef is also quite warm but there is a fantastic drive you can take. A high clearance vehicle is best but we saw passenger vehicles making the trip which is down a well graded dirt road. Ask the rangers about how to do the Cathedral Valley drive. There may be water running over the road is some places but it is only a few inches deep. Walk across the water first before driving. This is an all day trip. Check out this gallery:
My photosAnd this is what you see at the end:
This is the sign at the beginning of the road: