Usually, drink rounds increased in 'fanciness'--21 party at all houses was water and soda/diet soda, 12 party was usually something a little fancier (like blue punch or whatever for adpi), 8 party was something even fancier (hot chocolate with a chocolate spoon with peppermint), and 3 party was sparkling cider.
You can carry water or something in your bag, usually if you have more than a 1 round break between parties you go back to whatever meeting place your rho gamma assigns (union, library, etc.) to hang out. It's not the hunger games
ETA: well it is the hunger games, but not because of drinks!