As a resident of Las Vegas I concur in Thataway's assessment of summer at Lake Powell -- HOT! OK for a few days but it will wear you down pretty quickly.
How about a trip north or south along "I 5 corridor" -- Puget Sound, Columbia River, Willamette River, Lake Shasta, SF Bay and the Delta, Lake Berryessa (wine tasting too), Lake Tahoe and other lakes east of the mountains -- Paulina, East Lake in Oregon, etc -- then Lakes Mead and Mojave/Mohave if you need a dose of heat.
http://www.oregonphotos.com/pagethirty-Z.html Paulina and East Lakes
http://goberryessa.com/
http://www.tahoe.com/ -- it will be warmer in summer 8>))
Jealous--
Bill Uffelman
Las Vegas NV