Norton Common is a local nature reserve just north of Letchworth's centre, with grassland, woodland, and informal paths. It is not wilderness, but it is an important part of the town's green network and a good place to escape the planned geometry for a while.
Come here for a low-key walk rather than a major attraction. The common helps explain why Letchworth can feel roomier than its commuter-town neighbours, even when the architecture is not especially grand.