Anything Goes Party, for summer time fun.  Years ago I had a combined birthday party for my then 9 year old daughter and 6 year old son. We used survey tape to section off our back yard into specific areas for each activity.  We had approximately 20 children attend the party.  All the children were instructed to wear bathing suits so they could be hosed off as needed. We had one section for catsup battles, using watered down catsup in squirt bottles.  Another section was for a mashed potato/English pea war - I used instant potatoes and a few huge cans of peas and had it in large tubs. Another section was a water slide ("Slip'N'Slide") positioned on a slope in the lawn.  Another section was the children's swing-set where we had a wading-pool full of Jell-O for the children to slide into from the swing-set. There were more sections, but I can't remember them all right now - use your imagination...  Parents were welcome to attend and we had the patio area sectioned off as a "safe zone". My husband and I had prepared a "jail" out of a huge washing-machine box, cutting a window with bars and decorating the jail.  When the guests arrived the safety rules were explained to them all, basic safety rules of no squirting into eyes, etc., and punishment for transgressions was to be 10 minutes of "jail time" away from the fun.  After the fun and games was over we hosed all the children down and sat under the trees eating watermelons.  As for clean-up, we just hosed the lawn down too. The children were exceptionally well behaved and not one person had to visit the "jail".  I bought most of the needed supplies at Sam's Warehouse in restaurant-sized cans so the expense was not bad.  My daughter and son are now 24 and 21 years of age. They still speak of this party as being the best of their life and so do their friends.