Fairlands Valley Park is Stevenage's biggest and most useful green space, a broad valley park with lakes, paths, play areas, and watersports. It softens the town considerably, especially if you have arrived expecting only roads and precincts.
The park is best for a walk, a family break, or a bit of fresh air between trains and town-centre errands. It is not wild countryside, but it does exactly what a good new-town park should do: give residents and visitors room to breathe.