Oh please, please, please don't let it rain.
This was last year. It was a "Build Your Own Ice Cream Sundae" Party. A trip to Home Depot for a roll of construction tape that we put everywhere, a dozen balloons, and a few signs seemed to do the trick. We decorated our toolbelt aprons and had a scavenger hunt to fill them up (with kidsters' play tools).
Then we played Stick The Tool In The Toolbox. I put some paper behind play goggles as the blindfold. (Don't you just love duct tape?) The pinata didn't work out so well, as the first child (mine) pulled the magic string and opened it up. Instead of a five minute time killer it was over in one. So we wont be doing that again this year.
Oh please, please, please don't let it rain!