Ahh! Maine was awesome. It was a little weird taking a solo road trip, but I think I'd recommend it.
My best friend took a solo trip up to Washington last summer and loved that as well. If you can't get to Maine, maybe you could still wander around some other awesome states...?

There are definitely a LOT of things I should have done before grad school. But a quick road trip really only takes a few days; just take off a Friday and a Monday and go see something!