Acton Depot of the London Transport Museum on one of their open days and since then we've been going back twice yearly, at each of their open days. They really are worth going along to even if it's just out of a vague curiosity with that tube thing you get every day and how it came about. The next one is the 12th and 13th March this year.
What did the first trains look like? How were the tunnels built? Where did the people getting the trains go to?
All of that and more is on offer, including the chance to actually get inside some of the old rolling stock. Outside they usually have a mini steam railway chugging up and down and for kids you can't get better than that.
All the details are here but take it from one who's been - it's well worth a visit.