Technetium - you could consider going on a road-trip of that entire area. Even though it's summer, you can find many places that aren't 110 degrees every day.
You could fly into vegas and rent a car there (cheaper than you may think) and visit:
- Zion (backpack the West Rim Trail - 80s)
- Bryce (generally 80-90s in summer)
- Escalante (90s-100s)
- Arches (90s-100s)
- Grand Canyon, North Rim (80s-90s)
- Sierras (70s-90s I think, not sure)
- Capital Reef (90s-100s)
All have awesome day hikes and overnights (except for Arches, not much for overnight hikes there)