DSs aged 6 and 4 are terribly excited by mazes. I am terribly excited by any activity which gets boys running around outside in the fresh air.
We have been to Hampton Court Palace (a bit small, not much of a challenge even for beginners) and Leeds Castle in Kent (fab, hard and with an underground tunnel as its way out). The Leeds Castle ticket lets you in for 12 months so we will go back there as it was not too cheap.
Any other suggestions? Near south west London is great, but they were happy with nearly 2 hours in the car to get to Leeds Castle, so all ideas are welcome!