There is a $1/1 McDonald’s McCafe Lemonade coupon available on their website. (Which means it will be good at all McDonald’s.)
There are also some rumors going around that some stores are selling the McCafe Strawberry Lemonade for $1.00. I would either call ahead to double check before you go, because each store is allowed to determine their own store pricing, but a free strawberry lemonade would be awesome.