Some helpful things my best man did for me at my wedding last year:
-Coordinated with the groomsman for bachelor party. He simply asked what I wanted to do and took it from there.
-Paid me upfront for all the groomsman's clothing. I have one groomsman who was out of cell range for weeks at a time. Another who was terrible at responding to requests. This was super helpful and something he offered because he was financially able to. Not a requirement in my book, but man it was nice of him to offer and do it.
-When people (on my side) started coming into town the week of the wedding, he was their point person when they had questions. So if people had a question about location, or someone cancelled last minute, wanted to bring a plus one, etc, they talked to him. He relayed it to me when I had a chance to breathe.
-He waited until after the wedding, at the after reception party to get wasted. Also very helpful and deserved.
Just some of the highlights I can remember.