An on-site museum of the WW2 Enigma code breaking activities, where they happened. This is where Alan Turing worked. Live demonstrations of the 1940s eras computer "bombes".
But, perhaps, most of all:
A chance to visit a historic site.. the activities there could very well have decided the outcome of WW2.
There's also a neat live retrocomputing display of early home computers. And it also happens to be a very lovely area to hang around in - nice buildings, park areas, ponds etc.