4th Trip--What Should I See?

My personal favourite is Hampton Court Palace. Easy to get to, loads to see, beautiful grounds and gardens. You can easspend a day there.

I realize these are not unique, but I think your parents would enjoy them: Churchill War Rooms, if they are at all into history. Bletchley Park, again for the WWII history. Westminster Abbey and/or St Paul's for the utter majesty and again, the history. Is mobility an issue at all? That may help others make suggestions, particularly for unique things to do that you'll enjoy too.

If they like royal history, Kensington Palace hits a lot of it. Plus it's at the edge of Hyde Park and has a restaurant attached. Close by is the Natural History Museum and V&A Museum. I agree with the above recommendation for Hampton Court; take a taxi to make it easy for them.

There should be plays they will like, plus street performers in Covent Gardens. If your mom's a shopper, a stroll through Harrods might interest her.

If they want to get out of London for a day to see the countryside, look at Rabbie's Tours. They have two one-day tours from London. https://www.rabbies.com/en

Enjoy!

No mobility issues.

Open to any and all suggestions!

They might like the British Museum and/or the Victoria and Albert. The lunch area at V and A is beautiful.

Maybe a Thames River cruise to Greenwich?

Greenwich Observatory and nearby Royal Naval Academy Museum. Take a river cruise to get there.
Use the London Underground to get back. Interesting day out.

Hampton Court Palace
Churchill War Rooms
Bletchley Park

Train to Whitsable or Rye or Cambridge or Oxford

EDIT: definitely see https://witnesscountyhall.com/

A trip out to the Fens is likely to be something new to you, maybe visiting the Colleges at Cambridge and a gentle walk down the Backs, and how about an Iron Age settlement when you are in the Fens? In fact, National Trust Wicken Fen has lots to see...