Did you know you can walk from one side of the Thames to the other, underneath the water? There is something called the Greenwich Foot Tunnel. I definitely recommend going to Greenwich for this alone (though there is plenty of other things to do there).
The tunnel starts in Greenwich and crosses underneath the Thames.
It is super fun and free. The access is in Greenwich and near the Cutty Sark clipper ship (which we did not visit since it wasn’t open). There is a humongous elevator which takes you down to riverbed level. If the elevator isn’t working there are also stairs!
The tunnel itself has an eerie sci-fi feel to it.
Sections were damaged by bombs in the second world war and you can see the repairs.