Although it isn't a major holiday by any means, St. Patrick's Day can be a lot of fun for kids, and educational too. Learn about the history and culture of Ireland, and the origins of the holiday, or maybe write a limerick or two to pull in some language arts to your lesson.
A great exercise in creativity would be to have the kids design their own leprechaun trap. Give them some "found materials" like old egg or milk cartons, twigs, yarn, popsicle sticks, newspapers, etc. and see what they come up with to trap a leprechaun. You can really get some fascinating contraptions from this project and the kids love it!
Have a St. Patrick's Day party to celebrate the day. You never know, maybe a leprechaun will even attend!