Activity: Covent Garden. Tube Stop: Covent Garden Cost: Free. Hours: 10-8 pm.
Walk around Covent Garden and see if you can find some free live opera performances. Covent Garden has long been a hub for live music, but with VERY talented musicians. Even if you don't come on a day when someone is playing, it's still a great place to hang out, get dinner, and maybe catch an evening show.
There are also some cute shops in this area, but I mostly go for the street musicians.