We just did a "Mad Science" party for my DS's 6th birthday. It was a HUGE hit. In fact, one of the moms called me afterwards to tell me that her 9yo son said it was the best b-day party he'd ever been to.
I made the kids lab coats out of white garbage bags and a sharpie. Wrote out nametags of famous scientists on them (ie Isaac Newton, Einstein, Jane Goodall...) and let them pick who they wanted to be for the party. DH borrowed lab glasses from work, so the kids were "in character". I bought Ehrlenmyer flasks off ebay for drinking glasses. And we got 10lbs of dry ice pellets. They were amazed by the bubbling, steaming "green concoction" (Lime Kool-Aid), the bubbles that burst into "steam" (hot water, dish soap and dry ice). We did other experiments (made slime with laundry starch and glue, did some growing crystals, made flames come out of an orange) and the biggest hit was when we did Diet Coke and Mentos (which must be done outdoors and while wearing the lab gear!). DS's volcano cake was also a big hit. I used my Barbie-like Doll pan to bake the chocolate mountain, put a tall shot glass in it and then covered it with cookie and cake crumbs. I dripped red icing down the sides to look like lava. When we sang Happy Birthday, we put a mixture of dry ice, cream, sugar and red food coloring so it actually oozed red cream and when the bubbles burst, "steam" came out. We lit 6 sparklers instead of candles + a #6 candle for DS to blow out. We have the thing on video and it is really impressive.
The only downside is that it required a lot of parental prep and involvement, but in the end it was worth it. He got the party of his dreams, we spent less $ than going to Chuck E Cheese, and I don't think he'll be jealous when his sister gets to spend her 8th birthday at WDW in 2 weeks.
