What to do with my family in London?

I'm not sure this will really work here, but I think recommendations from HN readers will be a lot more relevant to me than I'll get from guide books and online resources. I'm going to be in London with my family (wife, 18- and 13-year old daughters), and looking for suggestions for interesting, non-obvious, non touristy things to do, especially evening entertainment. Pretty much any music but classical and musicals would work for us. The older kid especially loves 40s-50s popular music.