When I visited the UK a few weeks back, one thing I really took advantage of with my brother was the public footpath system. Basically, you can go for walks in the countryside almost anywhere - you drive about 10 miles away from the city, and you're out in the most beautiful countryside, and you have a right to walk in it, because of the system of public footpaths. If you've never experienced it, you just have no idea what you're missing.
Walking in the city is OK, but doing it in the countryside is an absolute joy.