I visited Bletchley Park in 2003 for a BCS meeting, and last weekend I finally managed to get back there for a day trip. Obviously a lot has changed in the near-decade since my first visit. There's even more to see, too much for one day in fact, so it's just as well that your entrance fee now gives you a year-long season ticket, so you can go back as many times as you like to see all of it.
Colossus (there is a separate charge for this in addition to your admission fee).
There's also a great cafe in Hut 4 for your morning and afternoon cuppa, and a variety of lunch options as well, a great way of keeping the hunger at bay and providing some more support for Bletchley Park at the same time. Cake was extensively tested, and found to be very good.
On this trip there wasn't time for everything, so the Churchill Collection and the National Museum of Computing will have to wait for next time, along with a longer look around the Block-B Exhibition Centre.
There's a few photos here.