Another vote for the Jurassic Coast, I would choose Lyme Regis – you can find fossils a few mins walk from the town, if you get the right time for the tide and good for a wander around.
Hexham I would also second, good for a couple of hours and cup of tea, we went there this year and then visited nearby Vindolanda by Hadrian’s wall, amazing to walk down the same streets on the same flagstones as people did 2,000 years ago.
In both cases reasonable weather helps!
If you go to London, IMHO it’s well worth doing one of the tourist bus rides.
At least that’s my idea of a good time…