We start off building traps the day before (gotta get them started today! it's going to be a bigger engineering feat this year). We build a leprechaun trap, put shiny sparkling things in it to entice the leprechaun, and when it's tripped (never catch the sucker), he leaves skittles and chocolate gold coins plus 2 real gold dollar coins skattered about as he escapes.
He also playes tricks in the house like turning toilet water green (put some food coloring drops in the tank, and it changes colors the first time they flush (unless your toilet leaks, then it'll be green when they see it). He turns milk green, eggs green, etc. He'll often do things like move furniture into weird places, make the TV speak in Spanish all the time (we have SAP on our back television, and I might send them back to watch a video for rest time). He leaves foot prints all over the house (I just use some green tempura paint and paint little footprints across the appliances and other surfaces that will clean off.
We bake Irish soda bread, and I usually try to have a corned beef brisket in the crock (you can get them pre-pared at most grocery stores this week), and we do mashed potatoes since cooked cabbage does not happen at our house.
We'll also do a little map work, craft and study on Ireland's history and St. Patrick's story that day.
Here are some ideas
And I'm thinking of trying this one tonight while the kids are gone to TKD:
Blarney Stone