It really depends on how long you have for this trip. I don't particularly like to travel when school is out, not because I don't like kids but I don't like crowds. So if time was unlimited I'd start March heading west along the southern route. Check out the Grand Canyon and and they Yosemite, and the Redwoods and Oregon Coast and Olympic 101 loop and then over Hwy 2 to Glacier and then down to Yellowstone and the Tetons.