I agree, Grand Canyon is waay too hot during the summer. Head for the hills!
#1: Glacier National Park grand loop
-Highline Trail, Stoney Indian pass, Ptarmigan Tunnel, Piegan Pass, Gunsight Pass, Sperry
90 miles, advance permits required. Mountain scenery, lakes, fishing, waterfalls, opportunity for scrambling and peak bagging. Ideal time is mid-July for max wildflowers and waterfalls.
#2: southern JMT
-South Lake to Whitney Portal
100ish miles, the best alpine scenery in the lower 48, high lake fishing, great weather. Ideal time is more late July to early August when the snow and bugs are mostly gone.
#3: south Yellowstone grand tour
-Thorofare, Trail Creek, Heart Lake, Lewis Lake, south Shoshone Lake, Lone Star Geyser, Old Faithful
70ish (?) miles, geysers and thermal wonders, wildlife, the largest alpine lake in the US, huge scenery, legendary trout fishing. Early to mid August once the rivers have fallen and the bugs are a little more mellow.