Every year we read the Legend of the Leprechauns poem and our kids decorate a box or bag to leave out in the moonlight the night before St. Patrick's Day. In the morning they have to follow the trail of greenery (left behind by the leprechauns) to find their bags filled with money, treats and gifts.
Now that they are older we are giving them money and movies passes instead of little toys and treats.
I put up our St. Patrick's Day decorations after they go to bed the night before to add to the excitement when they come down the stairs on St. Patrick's Day morning.
We always have something fun for breakfast like green shamrock shaped waffles or green pancakes.
...and we have to send them to school wearing green so they won't get pinched! :)