Where in Hawai'i have you been? The Big Island is pretty different from Oahu (the only 2 I've been to).
Lassen National Park is awesome and has a lot to do and see. If you can handle a few extra hours on the road, you could do Lassen and Crater Lake (in Oregon) in one three-week trip. They both were hits with my quirky kid. You likely will see snow, even in August, which is kind of fun. Lassen often has a lot of science events going on in the summer; you can check the website. We've been lucky to catch astronomy events and a hands-on program about the chemistry of the fumaroles with real-life scientists (including a xenobiologist, which I didn't even know was a real thing).
DS '04 "Boogaboo"
DD '08 "Lilybear"