The diet lemonade is quite nice. No need to get a shake! :-)
have you had their shakes though? they are soooooo GOOD! peach in the summer, their banana pudding/vanilla wafer one is also excellent and the chocolate candy/cane one at Christmas is extremely good too. I am not a milkshake person generally, never get them anywhere else, but the special seasonal ones they do are worth giving up a MEAL for!