OK, so I have 2 minutes on my PC..
We had ~20 6 and 7 yos, and we did the following:
Invitations sent out in plastic test tubes like these (the design had bubbles all over - I can send you a copy if you want)
Activities were:
Engineering: build the best structure you can with a given number of wooden coffee stirrers, marshmallows and cheese puffs (wotsits). We "judged" them while they were eating, and awarded each one a prize (highest, widest, messiest, most creative, etc, etc) - we did the whole thing on paper tablecloths, so we drew them a circle to work in, wrote their name next to it, and wrote the award they had won on there too.
Physics: shadow lanterns, using lantern kits which I cannot now find, but you could do it with paper bags and LED tealights - get a bumper pack of foam stickers from hobbycraft or wherever, get them to stick them all on and then pop an LED tealight in. This works best if it is going to be dark by the end of the party. Alternatively, you could get something like this?
Chemistry: We made silly putty type stuff, you can also make "goo" - I can search out some instructions online if you are interested. Either one needs borax, which you may have to buy online as it is quite hard to come by now.
Finale: Definitely requires outdoor space - we got one of these and lots of bottles of different coloured cheap and nasty fizzy pop, and turned it into an "experiment" by getting the children to guess which one would go highest.
If you are seriously nuts like us - dry ice bubbles - like this or this. Again, the challenge is in sourcing the dry ice - if you live anywhere near Heathrow I can point you at the people I used (we don't - someone had to pick it up for us!)
If you explore those last two sites, you will find lots of other fun experiments which you might prefer. If you want to PM me with an email address, I also have another whole set of experiments which another MNer used for her DD's science party and sent to me.
If you want general tips on party theming, food, cake, etc for a science party then feel free to head over to this thread - I used dry ice in the cake too