In the UK, at least?

So there's been quite a few travel threads recently, what with summer coming, however I'm much too skint too be going anywhere.

However, I do get free rail travel, which is amazing, but I've never really taken enough advantage of, so.... where should I go? I'm thinking of a few days travelling round the UK, using the sleeper trains at night.

Any recommendations of pretty routes, nice cheap place to see and the like? I've never really seen much of the lakes or Scotland, but apart from that I'm open to suggestions....