What to do in London on a rainy day?

Travel Info
I've been studying at Sussex Uni for the year, so I'm familiar with Brighton (also spent a good deal of time in Worthing and Lewes) but my parents are coming to visit in about a week. They are staying in London for the first couple days and want me to show them around. I have some of the most typical sights planned (Tower of London, London Eye etc.) but since the weather has been so wet... I was wondering if anyone had any good suggestions for what to do if it is pouring down rain. Thanks!

Travel Tips

London has about more museums than any other place in the world. If you can find one they like they might spend a whole day just there:
http://www.londonnet.co.uk/ln/guide/abou...

Shopping, there are malls, covert markets and department stores, most of which are just one quick dash from an underground station.

Join a bus tour, there are many and not all are open top ;-)
If you are familiar with the town you can even take a normal bus ride and supply commentary yourself.
There are several boat tours on the river as well as at least one on the canals.

As said, go to one of the shows, if you get the left-over tickets that should not be too expensive, or take them to a pub for a meal and the evening.

Contact the tourist information office, on-line or go to the office itself, they have even more ideas:
http://www.visitlondon.com/
Others

Ask question on Yahoo answers
Buy an umbrella. It will come in handy for most of the year!
watch british men getting drunk in pubs, south asians speaking in their funny accent in some south asian place, go to a london broadway type show, the english countryside must look beautiful in the rain. i have never been to london.i am east indian by the way. ;)
well u could take them to some fancy dinner they have never been too. then you could take them shopping for clothes and things that they want to take back with them. :]