I would just go to a pub. No tour, no walking thing... not that those probably aren't cool - but the absolutely neat thing about pubs is that you can meet actual people and talk to them! Example - in January of this year, we settled in at a pub by Trafalgar Square (but not on a heavily touristed street) and wound up talking to a charming couple (both London natives) who gave us super advice about what to see and do - and what to avoid - around Charing Cross, advice about the best bus to take from the Stables Market, and tips on a great chippie a few blocks away. More importantly, we also had a really meaty discussion about politics, which was kind of awesome, since we got the non-US perspective on a bunch of interesting stuff. Then, to cap it off, the husband, who was facing the window, yelped, "Goodness me it's a Chelsea Pensioner!!!!!!" So my husband got to see a real live Chelsea Pensioner. Which is kind of a huge deal. Anyway, that wouldn't have happened if we hadn't gone down the pub, on our own, and been open to a conversational gambit. Oh - and if you choose the right pub, you can also get a good meal.