You could easily spend 3-4 weeks just doing California and still not see everything. Trying to extend yourself by driving to Montana to visit Yellowstone while touring California would be a stretch. A fun tour would be up Hwy 395... you can get into the back of Yosemite at Lee Vining. Spend some time there... but beware of Summer crowds. When you leave, take Hwy 49 north through all the old Gold Rush country. At Placerville, you could head east to Lake Tahoe for a visit, then back into Northern Caliif and eventually over to the coast for a trip back south on Hwy 1. You'd have smiles on your faces the whole trip!