August is great in Tahoe, although its really warm in Reno at that time of year, but at that point the lake is almost swimmable (its a very cold alpine lake).
If they are going in early August, Hot August Nights is in town and things are a little crazy and can be expensive since its a big event. Its a ton of fun if they are at all interested in old cars.
What about Montana or Wyoming I bet you they could find some great little cabin type resorts that would be beautiful.
We were also looking in the US for honeymoon spots for a while and there are so really pretty bed and breakfast locations all up and down the east coast, Savannah or anywhere in coastal South Carolina would be pretty (but still far enough away that you won't feel like you want to visit).
Hmm let me think what else, I also agree with the others about Maine or other parts of New England would be great at that time of year.
Oh and about 40 minutes west of the DC area we have a great wine country in Virginia and some beautiful areas around VA, that's always an option.