Great Central Railway at Loughborough:
http://www.gcrailway.co.uk/
You get a decent ride on a steam train, dinner on board if you wish, and you also get to look around the engine sheds and have a wander around the platforms. It's one of the few things everyone can enjoy no matter how old they are.
You might also want to check out Crich Tramway Village in Derbyshire, near to Matlock. Similar thing, but has trams instead and is much bigger. It's also one of the few places, I've noticed, that you can take family dog.
http://www.tramway.co.uk/