Milton Keynes Museum - a hands on history place that you can try toast cooked on the range, sit in an old fashioned school room, try victorian parlour games and their pianola, work telphone exchanges with phones from the very old to the very new whilst watching the innards at work, walk through and visit old shops, loads of old farming and vehicle stuff to climb over and investigate. Oh and a good tea room at the end :)
There's even a steam(?) boat parked in the car park - which, given how far MK is from the sea, is pretty impressive!
Our 6 and 3 year old ds's loved it - and dh and I found it pretty interesting too!