Natural History Museum
by YLM
(London, England)
This is one of my favorite places to take the kids when it's raining. Fortunately I live quite close so it's a viable alternative to the hideous jungle gym things that have sprouted up everywhere (my idea of HELL, kids idea of HEAVEN - Natural History Museum is where we compromise...) but it's well worth a trip however far you have to travel to get there.
A word of warning though - the Dinosaur Zone is very realistic (well, if you're under the age of 6...) and if your kids are of a sensitive disposition then be prepared for tears! The bug zone is also pretty cool and there are lots of interactive exhibits (some more technologically advanced than others) which kids go crazy over. My two are also fascinated by the small collection of stuffed animals, but I think this is more morbid curiosity than anything else ("Are they dead? Why are they dead? Are you sure they're dead? How were they killed - did a baddy get them? Why have they been stuffed? Where are their mummies and daddies? etc).
The other thing is that it can get uncomfortably packed so try to get there as early as possible or on a weekday if you can to avoid the crowds, especially at the weekend and during school holidays. Obviously I am not suggesting that you and your child play hooky from school (that would be terribly irresponsible, wouldn't it?) but the earlier the better...
Oh, and take your own packed lun