You've had good suggestions above.
I'm pleased to see Winchester on your list. I don't think it gets enough of a shout out here. Easy train trip, and there's the castle, the cathedral and Winchester college (posh boys school, that you can book to visit.) Although the cathedral loses to Salisbury on the outside, IMO it is way more interesting on the inside. But if you go to Stonehenge, Salisbury pairs much better with that. Its a bit what appeals to you, I found Stonehenge boring, but am probably out voted by those who find if fascinating.
I would drop Liverpool as a day trip option. Just about doable as a day trip, but with so many options much easier, unless you really, really want to do it I'd pass. And I love Liverpool.
Other options, depending on interests. You don't mention the Harry Potter studio tour. I've never been, but everyone I know who has seem to really enjoy it (even the non-Potterheads). Easy train from Euston. But you have to book the tour way in advance.
There is also the WW2 code breakers and early computer site at Bletchey Park. I found this fascinating. Easy trip to get there from Euston.
Also Hampton Court Palace. You could travel part way or one way there along the Thames. It is also near Richmond (Ted Lasso.
Or Greenwich. Again possible to travel along the Thames to get there. Maritime Museum, Cutty Sark ship. And the Prime Meridian at Greenwich Observatory.
Edit to add. Look also at www.walks.com They do some day trips from London. I think they do Cambridge and the Cotswolds, maybe Oxford. Check if they have a trip when you are here, there have blue badge guides and their trips are very well organised. But their options wont show on their site until nearer the time.