I've recommended Bistro 1 lots of times here (as have several others) for decent food at sensible prices and the feedback has always been positive. They've got 3 branches but I always use the Covent Garden one:
http://bistro1.co.uk/
(If the weather's fine, simply watching the Covent Garden street entertainers - or the wonderful opera singers - can be fun. It's also free unless, of course, you choose to contribute something).
Otherwise, any of the South Kensington museums are well worth visiting. The Science Museum, the Natural History Museum and the V&A are all grouped closely together. They're all free.
A less well-known, but still fascinating museum, is the Museum of London Docklands. (Again, free!). It's close to West India Quay DLR station and next door to the classiest Wetherspoon's pub that you're ever likely to encounter.
http://www.jdwetherspoon.co.uk/home/pubs/the-ledger-building
(There are also some other good eating places there as well)