Those of you that are coming for more than one day and planning on staying, watch out as there really is very little if not nothing to do in Bletchley. It's just charity shops and Pound Savers. Milton Keynes is your best bet really (10 mins max from Bletchley), but the bits worth checking out are pretty spread out so bring some wheels if you can.
Best to do some research as I don't know what you're all into (apart from the obvious). Having said that...
Shopping - MK is excellent if you like this - the Gamestation is good as well (good window display which always has the best stuff in it).
Walking - believe it or not, MK has a lot of decent routes which will all make you think twice about what you have been told of the man made mesh of roundabouts. Maps are available if you're into this.
Drinking - there are some pubs in Bletchley but I can't vouch for them. Bars in MK are plentiful but not my cup of tea at all. Although I am from Bucks, I live in Yorkshire and enjoy cask ale in real pubs. In fact, I probably live in Yorkshire BECAUSE I enjoy cask ale (crikey!). So you can probably guess what I think of wine bars...
Countryside - lots of it within a few miles. Classic Buckinghamshire. If you have a car, what's stopping you? The National Trust website is worth a look.
Activities - in MK there is an Xscape, so you can play glow in the dark bowling, climb an indoor wall, go skiing on an indoor slope with real snow and experience sky diving indoors (look it up). None of which is cheap, but all GOOD!
MK Dons are also based here (between Bletchley and MK) - the stadium is excellent (new a few years ago) and they're actually pretty good. It's the weekend so you'll get a game.
There's a overground (in the trees) obstacle course as well - like a mini Go Ape. Can't remember what it's called.
Eating - loads of places. Best food is found in out of town village pubs imo. I said pub again...
Cinema - two in MK - both alright - overpriced as always. Neither show anything other than latest movies (so no arthouse, classics or world cin).
Erm, that's all that comes to mind at the moment. My mum makes an ace cup of tea as well.