Arthur Figgis wrote:
There are various heritage railways - the Mid Hants and (more awkwardly)
the Bluebell are reachable from London.

If you're willing to get out of town, how about the Isle of Wight? 1.5
hours on the train to Portsmouth from Waterloo, then 15 mins on the ferry
(catamaran, I think). There are 1938 tube trains connecting with the ferry
and running down the east side of the Island in their retirement, and these
connect with the Isle of Wight steam railway.

At Portsmouth there's also the historic dockyard: Mary Rose (1510), HMS
Victory (Nelson's ship from Trafalgar, 1805), HMS Warrior 1860 (first
British ironclad). And plenty of other current naval ships (there's perhaps
a better view of these from the Continental Ferry Terminal, but I'm not sure
it would be a terribly good idea to wander around there snapping away).

That's probably too much to fit into a day...

Theo