During the summer, anything below 7000 feet is going to be hot. As long as you know that and are prepared for the heat, go get 'em!
Zion is particularly great for young bikers -- you can ride your bike all the way up the canyon (on pavement) and then coast back down, enjoying the scenery. We have done that many times -- it never gets old. Stop for an ice cream at the lodge on the way up! And there is no traffic on the road -- cars are banned, and the buses are very courteous.
Kodachrome State Park is not too far from Bryce -- great campground, and great scenery at the state park itself.
Good luck -- I sure hope we all get to travel this summer.
