Leicester's National Space Centre is the UK's largest space attraction and genuinely worth the trip even if you're not particularly interested in space. The building itself is striking — a 42-metre titanium-clad tower of an inflatable structure visible from across the city — and the exhibitions inside cover the full spectrum from ancient astronomy to the current state of commercial spaceflight.
Six galleries cover rockets, astronauts, satellites, and the universe itself, with a mix of interactive exhibits and genuine artefacts. The Sir Patrick Moore Planetarium shows change regularly, covering topics from the history of the night sky to the search for exoplanets. It's excellent for children, but adults who come expecting a kids' museum are pleasantly surprised. Allow 3-4 hours.