We had a rainbow treasure hunt at the party for 4 year olds! They loved it. I had them find an item of each color and bring it to me. for every item they found I gave them a gold chocolate coin. This was my take on this idea - http://familyfun.go.com/printables/st-patricks-day-treasure-hunt-1036011/
OMG I love this idea. Lead him on a treasure hunt through his car to the trunk for a surprise date night at a drive in movie or just a movie and everything you'll need. Free printable a and everything.