I once vacationed in the highlands. It was beautiful and the people were fantastic, but it was the wrong time of year and it was very windy and wet.

Having said that, I second (or third or fourth) going to Northern California. Lots to do, beautiful scenery, plentiful road and mountain biking, and not so hot. I was going to recommend the Texas Hill Country or the Carolinas, but agree that September is just too hot and muggy if you're not used to it.