Hands-on museums and science centres. Several of the best ones are free.
The full Old Trafford works: three floors of museum, then the players' tunnel, both dressing rooms and the dugouts on a 70 minute guided tour. Plan around the fixture list, because it all shuts on home match days.
Free, indoors and better than it sounds: interactive science aimed at younger children, glassblowing demos, a picnic area and a small playground outside.
Five floors, free, and it covers three things small children reliably love: giant dinosaurs, an aquarium and a space gallery. The best wet-weather day within half an hour.
Hands-on science galleries and a glass rooftop observatory over the Mersey. Small, cheap, indoors and close.
Working steam engines, the story of Cottonopolis, and hands-on galleries in the world's oldest surviving passenger railway station. Free, and the live demonstrations are the bit kids actually remember.
A T. rex skeleton cast, live frogs in the vivarium, Egyptian mummies and a genuinely good natural history floor. Free, university-run, and less overwhelming than the big London equivalents.
Shirts, trophies and match footage across six floors, with penalty shootout and skills games that football-mad kids refuse to leave. If yours do not care about football, skip it - it is exactly what it says.