If you like glaciers (and who doesn't?), I'd recommend Paradise Glacier on Mt. Rainier -- this photo was taken in late July a few years ago. It looks like we are advanced mountain climbers (we aren't!), but this location was an easy stroll from the lodge:

Click For Full-Size Image.
I would also recommend Mt. Lassen for pre-teens -- the walk through Bumpass Hell is amazing -- the mud geysers bubbling, the steam geysers whistling, very cinematic. (And Bumpass was the name of the poor guy who discovered this weird place. Wait till you find out what happened to him . . . . )