My kids love McDonald's Shamrock Shakes, so we decided to create our own version. I just kept it very simple (2 ingredients), but the kids were very pleased with the results.
Blend about 2 cups of Edy's Mint Chocolate Chip Ice cream with 1 cup of milk.
The little chocolate bits make it so much better than McDonald's. If you want a stronger mint flavor, add a little bit of peppermint extract. You could also top with whipped cream and a cherry to make it look more like McDonald's.