My daughter is very much into science: experiments, space, anything related to science. She asked for a science-themed birthday party.
I found a great website that had all the materials and detailed instructions for for some really cool experiments to do at the party - everything was very hands-on, so all the guests got to do something fun. Steve Spangler Science (website).
The mentos and diet coke explosion has become fairly familiar. I thought this would provide some excitement. I bought several 2 liters of Diet Coke and a pack of Mentos, so we could perform this a few times. Steve Spangler's website sells a contraption that makes it easy to drop the mentos into the bottle while allowing enough distance and time to run away before the soda volcano covers you.
Inside, we all played with sand that didn't get wet, blue goo worms that went from liquid to solid out-of-water, and tiny gelatinous beads that not only expanded significantly in water, but also took on the color properties of the water:
Also inside, we held a centrifugal force experiment with a bicycle wheel. By spinning the wheel and tilting it slightly, the person holding the wheel felt strong forces of movement associated with that.
Outside, we continued our fun with some Insta-Snow (from Spangler). Just add a small amount (I used plastic cups for each person), then combine with water to watch it grow instantly into snow. It felt exactly like snow! Amazing experiment - I highly recommend. (You just can't eat it). It only disappears as it dries out, but you can add more water later to bring it back.
Finally, we took a look at the moon - it was a perfect evening - through my father-in-law's telescope. It was the first time for most of the guests, so they loved it!
From not knowing how to pull off a "science" party to getting ideas from my father-in-law and some great ready-to-do and kid-friendly experiments from the Steve Spangler site, it turned into a successful evening and one that the kids talked about for months!