There are lots of great tours in Europe, and I hope I live long enough to do them all, but here's one I can recommend first hand.
http://www.activejourneys.com/tours.cfm?aid=31&tid=10
My wife and I did this one on our honeymoon, and it was fantastic.
It's very doable for almost anyone of any ability level.
I'd recommend doing it in late spring or early fall, as the pathway gets exremely busy in sumer, not to mention the heat.
The accommodations were first rate, lots of points of interest, and great food.
We went self guided, and were very pleased with our decision. That way, we were free to choose our own lunch stops, and our own pace. We watched the guided tours rushing to get the ride over with every day, and that just isn't what we were there for.