Ely, 25 kilometres north of Cambridge on the railway line, is a natural day trip: a small market town dominated by one of England's most extraordinary cathedrals. Ely Cathedral, begun in 1083, is most famous for its Octagon — the central tower that collapsed in 1322 and was replaced with a unique wooden octagonal lantern tower, one of the finest achievements of medieval engineering in England.
The train from Cambridge takes 20 minutes and runs frequently. The cathedral dominates the town from the moment you arrive; it is visible from miles away across the flat Fens. The Stained Glass Museum inside is worth the extra admission.