I have 2 huge plastic tubs, similar to the ones pictured earlier, that we use for drinks at parties. As long as we keep them in the shade, the ice will last for many hours. It depends on the temperature, of course, but we've never had our ice melt completely and the water (and drinks) go warm in less than 6-8 hours. Of course, you do have to take into account that you have to put the drinks on the ice at least an hour before the party starts, so that does eat into your time a little bit. You wouldn't have to do it as early if your drinks were already chilled, but you probably don't have that kind of refrigerator space before a big party.
Oriental Trading Company sells things that basically look like kiddie pools for holding drinks in a variety of themes, including graduation. I've never tried them, though. They look like they're so shallow that you would have to add new ice more frequently. You want something with less surface area.
I think that open bins of some sort are friendlier than coolers. People can tell by glancing which one has water, which one has soft drinks, etc. Coolers do last longer, but you need a lot to hold many drinks, and opening and closing them is a pain. I'd consider using them if the party was going to be an all-day affair, but for something that's going to be 4-6 hours, bins are a good choice.