One of the most popular booths at NGS seemed to be the 1940 U. S. Census Indexing booth. Anyone who stopped by and indexed two batches received a T-shirt. Several times I saw a line of people waiting their turn. Even on Saturday afternoon, less than two hours before the exhibit hall closed, every seat was filled.
Saturday afternoon indexing
If you haven't volunteered for indexing, it's not too late. Check out the 1940 U. S. Census Community Project for all the details.