Wow - that's a lot of kids for a 3 year old's party! We always followed the "year" rule - at 3, there are 3 kids invited, at 5, there are 5 kids invited, and so on. It's hard to organize anything for little kids. You could try some sort of very simple craft with adult help - sticking foam shapes on a foam picture frame, for example, and then e-mailing everyone a picture of their own child later on. There is a big difference between what 2.5 year olds can do and what 4 year olds can do. With the homemade pizzas, that's probably enough organized activity.
Otherwise I would sit in a circle and sing things like "The Wheels on the Bus" and other fingerplay songs that engage everyone, and otherwise let them just play with whatever toys they choose. They will not understand birthdays, and the little ones will not understand why your child gets presents that they don't. Sometimes that can get ugly! So I would just take a few pictures with the cake, and be done with it!
Congrats on your daughter's birthday!