Potlucks often run out of drinks and ice. I used to work at a place with a monthly Potluck and I became a friend to many by always bringing ice. I was told by a lot of people no one ever remembered it and it was much appreciated. Stop at a gas station day of and pick up two bags of ice and a bunch of sodas and maybe some fun drinks (Snapples etc.). Even if drinks are always taken, it can't hurt to bring more.

Or, alternatively, go to a grocery store and get a couple pies from the bakery and some ice cream. Who doesn't love a good dessert?