Well, first, I don't think it would be at all out of line to ask for RSVPs. That way you'll know how many to expect. (Around here it's more traditional to invite 5-10 closer friends, but I'm not sure what the customs are where you are
) I've found that Sam's Club (Cosco in your area?) has bakery goods and ice cream that really are good enough and cheap enough that it's often not worth trying to do homemade esp. since you feel the stress of a time crunch. If you're not a member, maybe you have a friend that is? Also, our Sam's lets you shop there without a membership, but it's a 10% premium -- it might still be worth it. I'm sure you'll get lots of good game ideas from others (my daughter is a bit young still). However, if it turns out that it's all girls who come, you might want to do a beaded (elastic) bracelet project. I think if you used the bigger beads I would work for 5 year olds? I think it also pretty cheap and kind of different.