Our last two or three trips, we've left a couple days open for serendipity. It's worked out WONDERFULLY. But we generally limit our travel to one part of a country and rent a car to maximize our ability to get places. (And this year we were in South Africa, so with English spoken everywhere it was even easier.) It also helps to travel off season, so you're not stuck hitting sold-out hotels, etc. Actually, I was thinking that next year we'll NOT plan for anything other than that we'll go for 2 weeks in May. Then, if we see some fantastic opportunity at a special rate, and we want to go, that's what we'll do. Not sure if I'd leave it TOTALLY to chance at the last minute though.

The thing about arriving a city on the train and "checking into a 2-star hotel", is first you have to find one! Personally there are many things I would rather do on vacation than spending time searching out a room for each night. This daily chore is the place where the fantasy meets reality.