Bletchley

If you are in the car you could pop into Woburn Abbey, a little country house of the Duke of Bedford. They also have a modest deer herd roaming wild and a small zoo.

They do a decent tea.

Without leaving the grounds of Bletchley Park you can also visit the National Computing Museum and see some real history.

Woburn Abbey is close, but I've no idea how easy it is to get between the two by public transport (I assume you won't be hiring a car if this is just a day trip). Perhaps a taxi wouldn't be too dear as its only ten miles. Incidentally despite the name it's not a religious building, but a stately home, famous for its garden and art collection, especially a lot from Canaletto for some reason

However, will you need a second sight? There's a lot to see at Bletchley, even more if you also visit the computing museum. Together with lunch, that might be enough for a day trip on its own. We spent four plus hours there a few years ago. Haven't been to Woburn in donkeys years but I guess it's not changed too much.

Edit - looks like Nigel got there first! But at least we're saying similar things.

It is a full day to go to Bletchley Park. I think we left London about 9 AM and did not get back until 4 PM or so what with touring the site, having lunch, and of course the transportation. We found a lovely Middle Eastern restaurant in town Pasha.

Explore the Chilterns

https://www.nationaltrust.org.uk/chilterns-countryside

St,

Bletchley Park is a large site and I found that it takes the better part of a day to see all the different huts to get a good sense of what took place there. I managed a short visit to the onsite Amateur Radio station but didn't have time for the computing museum. I'll have to make a return visit for that.

Bletchley is a must-see for students of WW2. It's a relatively new operation, in part because the code-breakers were sworn to 50 years of silence and many, especially the strong corps of women, were so true to the vow that they didn't tell their own children, never mind nosy historians. Try to go at a time when the replica of the huge code-breaking machine is being demonstrated. There is a basic tea room on site but the town is a short walk away with a couple of pubs and some simple restaurants for lunch. I went by train from central London, a pleasant if not very rapid commute.
I like Nigel's droll description of Woburn. He can correct me, but I think it set the model of turning the family mansion into a tourist attraction to help pay for upkeep on the ancestral pile.

I enjoyed Bletchley in 2013, but to me it’s worth between a 3 to 4 hours visit. I know some renovations have been completed so it may be worth a little longer now. We had a rental car so didn’t have to arrange public transport.

We have visited Bletchley twice in the past few years. We took a train from London and the train took us straight to Bletchley. It was so easy. We spent all day there. We took a guided tour and spent hours looking at the displays. We had a delicious, affordable lunch on site which helped to make for a great trip. I agree with previous comment to make sure you visit the museum of computers while there. We also loved the exhibit featuring the movie about Alan Turing. So fun!