Could you do some cooking with them? Could be rally messy but if you got some pita bread and got them to make their own pizzas they could thn do pass the parcel while their food cooks? cupcake decorating is another one which goes well, I make a couple of big batches of cupcakes, the $1 packet mix is fine as the fun is in the decorating more so than the eating, mini marshmallows,sprinkles and choc chips are good to decorate.

Do you have many adults to help? Could you do a make the best costume out of toilet paper and sticky tape ? Do you have craft gear? If you do get some boxes and split the boys into groups and get them to make robots, my brothers load doing this as kids

Pin the tail, although this normally doesn't take much time

Hope the party goes off well, bad weather and parties is not fun