We had Mad Scientist Sam, who started with a card trick, then went on to make everyone some slime, showed us how to get his friend Egbert the Egg into a glass beaker where the neck was too small, then did fun things with vinegar and baking powder, made a tornado in a bottle, handed out some diffraction glasses, set fire to some flash paper, before finishing up with a practical 'how to make candyfloss' session. All whilst telling a room of six year-olds the sciencey bits behind what was going on.
All entirely good fun, but I couldn't help thinking that nalsa would have made a far better job of it, and injected an entire order of magnitude more enthusiasm (and hand waving, balloon animals etc) into the proceedings.
The candyfloss was delicious though. And watching a room full of small people wearing diffraction glasses wandering around like they're stoned going 'whoah! the... colours!' is hilarious. One small child promptly announced that he was now invisible!!, and wandered off.
 Mad Science Parties!
 PVA glue, coloured paint, water, plus borax. Stir until gooey. Do not eat or get on the carpet.
 add one to the other and watch what happens. Put some vinegar in a test tube, some baking powder in a balloon, put the balloon over the end of the tube and shake the powder in. Great fun.