I too am a Diet Coke drinker, with lime. Sometimes I don't tip, if just grabbing a quick refreshment during a short stop-over in the club. If I have time, usually I order two right away, and sometimes down one while standing at the bar (of course, only if no one else in line) - then I tip $1. If I have a long layover and plan on a couple trips to the bar I'll tip $2 on the first visit.