Lake Mead, Lake Mead National Recreation Area, Mojave Desert, Nevada. I grew up just a few miles from this lake. Great times here under the Boulder (Hoover) Dam. Skiing, boating, camping!!! Great Lake to play on! miss it
Lake Powell, in Arizona and Utah. The second largest man-made lake in the US (the 1st is lake Mead). There are 96 major canyons to explore though you'll need a water craft for the majority of them since access is limited because there are few roads.