RMNP - amazing high alpine areaMt. Evans - almost 4000 m altitude, but I didn't like the rock up there that much...Tuolumne Meadows, Yosemite - ca 2500 m altitude, stunning scenery and some really perfect problemssome of the smaller crags in the bay area seem to be resonable in summer too. Lake tahoe has tons of great bouldering in summer and Squamish isn't that hot too.
The most popular areas are the Colorado areas mentioned above. Other good options would be black mountain or the palm springs tramway area, both in southern california.
Summer bouldering areas in the states