I also recommend breaking the kids up into smaller groups and do a 'round robin' type thing. No matter what theme you go with, have multiple games/activities planned and then let each group do them (group A will do a science thing, group B can decorate a pre-baked cookie, and group C can do somehting else. After 15-20 minutes, or whatever time you think the activity will take, then rotate.
We've done Pirate parties (each kid got an eye patch, bandana and a plastic hook or knife), we did a scavenger hunt that led them from clue to clue etc, to finally find a treasure chest.
We've done a Harry Potter party. We basically followed the line of the first book. Each kid got to go to Gringots to get their gold gallions, then onto Ollivanders get get a wand, then to Florish and Blots to get their spell books. Then onto school and each kid was sorted and given a badge for their new house. We did a potions class (we made 2 science experiments - one was drinkable, the other they got to take home). We did a charms class, and defense against the darks arts (this was a ghost pinata). This year we did a Minute to Win It pary.
I always find it easier to pick a theme and then the ideas seem to come pretty easily. Also, there is so much info on the web that when you pick a theme, just good "sports party" or whatever the idea is.
Have fun but for the it is easiest when I have too much planned so there is very little down time. With too many kids and down time, it can be hectic.
Enjoy!!!