What is there to do in London on a sunday afternoon/evening?

There are various theatre shows on. London Walks have a number of interesting places to wander around. The comedy store has a show. Final link are things happening on Sunday 24th, but you can look at other dates on the website.

explore and walk about

drink tea.

I think since it'd be your first day there and you'd be arriving later in the day, you should go to London Bridge (station) and take a stroll down the Thames down toward Tower Bridge and stop off at a bar/restaurant for a chillaxing drink and evening meal before heading back to your hotel, and then start off the next day refreshed and ready to take on all the exploring and sight seeing on Monday :)

London comedy store's Improv Night with the Comedy Store Players ( including my fav Paul Merton) ( if you enjoyed Whose Line is it Anyways on the Telly- they still have many of the people that did the UK version- get tickets early sunday night shows tend to sell out) The comedy store is right in the middle of Piccadilly circus- plenty of places to eat/shop before hand

It depends on your interests: Stroll around. Visit art galleries, or museums. Visit historic sites like the Tower of London. Visit Speakers Corner Marble Arch, a Sunday afternoon venue for anyone to speak to the public. Go to a pub or restaurant. Visit a park

I'd recommend you visit Brick Lane (take the tube to Liverpool Street or Aldgate East). There's a very laid back vibe, you can visit some boutiques, get a curry and end the day with a few beers in the many bars they've got in the area.