SprawlWalker creates UK urban navigational challenges for you to complete. Once you give it your local train stations 3 character identification code (available online, if you don't already know it) and the rough distance you would like to travel SpawlWalker finds another station for you to walk to, in range.
The idea is that you find your way to the target station by whatever means you see fit, then get a train back to your local station. Try setting yourself some rules before you start to add to the challenge (for example, not being allowed to look at a map). Try heading towards tall buildings, tracing traffic flows into and out of cities, it really is up to you.
If you like adventure and walking in urban areas, this really is the app for you.
Search terms: city,exploration,adventure,walking,rambling