Stevenage is rarely anyone's picture-postcard Hertfordshire stop, and it is better to be honest about that. It was Britain's first post-war new town, planned for growth after 1946, and much of its centre still has the practical, concrete, pedestrianised confidence of that era.
Look a little closer and the town becomes more interesting. The Six Hills beside the old Great North Road are Roman burial mounds hiding in plain sight, while St Nicholas' Church and the Old Town keep the pre-new-town settlement visible. The Stevenage Museum is useful for making sense of the abrupt leap from village to planned commuter town.
Stevenage works best as a short, curious stop between Hitchin, Letchworth Garden City, and London. Add Fairlands Valley Park if you need open space, or the Gordon Craig Theatre if you are staying locally in the evening.