They usually sell ears. They sell a pin year round. They used to sell a shirt every year but they didn't last year. There will probably be some special food, like a CS place might have a green cupcake or something for a few days.
Additionally, some of the DTD locations might have special menu items. Last year, the Disney Parks Blog posted a list of the special menu items for the DTD locations in early March. I would keep an eye on the Parks Blog to see what info they post.
There will also probably be some small amount of merchandise (some available year around, others released about now). There is a limited edition pin, but it's being released on February 18 and will likely be sold out before St. Patrick's Day.