if you can be outside, its great. get things like bubbles, sidewalk chalk, toys, etc. kids loooove those parachutes, you can get a cheap one at oriental trading.com, lots of craft ideas there too, i usually set up a craft table outside with some activities. its also great if there are kids who dont know everyone or just dont quite get into the groove with the other kids, it gives them something to do. if you have enough outdoor toys, they will occupy themselves. just remember plenty of snacks and drinks. my advice is that if you are gonna have water activities, be sure to let the parents know ahead of time that it will get messy so they will bring a change of clothes etc. have extra sunscreen on hand. if you get stuck inside and you dont want to let them run amok like i do, just set up a space for them with enough toys etc, crayola color wonder is great if you want to have indoor crafts but dont think you will be keeping a close eye on things. those foamie activites are great, no mess. scavenger hunt. those big balloons on the rubber band are always fun for all kids to bop each other with. get a little disco light and let them have a dance party. you probably have enough stuff around that you can set up a play classroom for them, chalkboard/easel, flash cards, construction paper and crafts, etc the older ones can "teach" the littles. check out wondertime magazine or family fun magazine, tons of ideas there. a tip, when you mix kids into an adult party, be really careful of the alcohol if you are having any. i had my kids at a new years party, mostly adults with about 10 kids as well. i walked into the "kid room" just as my then 3 year old son was sampling a drink that someone had left on a table. take whatever precautions you need to. have fun :)