London Day Trips

I don't want to say if your timing is right as you may want to see more or less than me and at a different speed.

transportation suggestions.
Bletchley Park - whole or half day? Tube from wherever you are staying to Euston (Victoria, Northern Lines) and upstairs to Euston Station. Or the Circle, Hammersmith and City, or Metropolitan Lines to Euston Square and a short walk to Euston. Or Thameslink to St Pancras International and a short walk or the Piccadilly Line to various change points including Kings Cross St Pancras.

Once at Euston take the first London Midland train which will stop at Bletchley. That would be services for Milton Keynes Central and many services to Northampton and Birmingham New Street. Do not take the xx:46 or the xx:49 neither of which stop at Bletchley. Still that is several per hour. Take the train to Bletchley, and when you leave the station cross the street and follow the road straight ahead for a short distance. At the "T" junction turn right and Bletchley Park is almost immediately on the left. Total walking from the train less than 10 minutes. Bletchley Park is a big place. Be sure to also see the National Museum of Computing, on the same site. Any train back south from Bletchley will get you to Euston.

Blenheim Palace - planning for a full day
take a tour from London, or a train from Paddington to Oxford and then a bus. The place is huge and I can't imagine you taking less than a whole day.

Harry Potter Studio Tour - can this be done in a half day?

Once at Euston take the first London Midland train which will stop at Watford Junction. That would be services for Tring, Milton Keynes Central and most services to Northampton and Birmingham New Street. That is several per hour. Take the train to Watford Junction, about 15 to 20 minutes. As you go outside the station cross the forecourt (watch for buses and taxis) to bus stop number 4 where the special bus takes you to the Studio Tour just a few minutes away.

The studio tour sells out and you need to get tickets in advance, fairly far in advance.

I bought a book a couple years ago "10 Best Day Trips from London" it had all the info I need to plan to get to these locations, what to see etc.
I see that now there are several books with similar titles. I see a couple of them at my local library, but they are also for sale on Amazon.
I would recommend looking at 1 or 2 of them for additional info.

You can't see all of Bletchley Park in a single day if you do it properly.

Blenheim is a full day too, as it's quite a long way out of London.

HP studio is c 3 hours actually there plus at least an hour there and another back. Tickets are essential and book a long way in advance if coming during school holidays.

Closer days out would be Greenwich, Hampton Court, Kew Gardens or a bit further, Canterbury, Leeds Castle or Windsor.

I recently visited the HP Studios and think it could be done in half a day if you don't plan on spending a ton of time there. My tour was scheduled for 2:30 and I left for the studios about 70 minutes before the start. I spent a good 3 hours there and was eating dinner back in Central London 7:30. I was able to ride the London Eye and go to King's Cross Station in the morning.

Also, it's very easy to get to! You can look on any thread here or on their website for directions. I was there about 15 minutes early for my start time, so it really doesn't take more than a hour to get to.

Although we enjoyed Blenheim Palace, have you considered Hampton Court? We preferred it, and you can go by boat for something different.