This page offers a good overview of humidity, with some links to experiments like creating your own psychrometer (which until this week I would have guessed had something to do with mind reading)--good for older kids, but you'll need two centigrade thermometers.
Here's a printable geared for grades 5-8 as well.
These questions from the BBC may stimulate thought in younger or older students. Here's an experiment to start early in the week about humidity, and another using pinecones to create your own hygrometer, which I know you've always wanted to do (I think there could fairly easily be a substitute for the plasticene).
Believe it or not, here's a (slightly or very corny--it's all relative) song about humidity.
The best find this week? Scholastic's Interactive Weather Maker, where your child adjusts the relative humidity and other factors and watches what it does to the weather in this little virtual neighborhood.