Try to get in an activity or something to break it all up. I was best man twice last summer, the first stag do we went drinking in Newcastle but to break up the drinking we did Zorb Football. Not something I'd never do normally but we all look back on it fondly.
The second was Baz's trip to Belgium, we took a train on the Saturday from Brussels to Bruges and got tickets to the Belgian Super cup which was very cool.
Meant that neither was just another day of drinking like you might do for a birthday.