If you've ever seen fans with their faces painted, then you know it's a fun way to express support for your favorite team. Using your team's colors, paint your face with a different color on each side. Men can go the extra step by painting their chests, as well. The more people involved in face painting, the funnier it will be. In addition to painting your face, you can dress from head to toe in team colors.
Halftime is midpoint in the game and teams get to take a break. Organize a group of people to put on a halftime sketch behaving as a team with no skills. Perform a 3-5 minute "mock" game with people purposely missing baskets and playing badly. Have a fake referee make bad calls and blow the whistle. Give the fake teams the same name as those playing the real game and entertain the crowd.
Foot Signing Contest
Contests are usually conducted during halftime, too. Have five people remove their shoes and socks and go into the crowd with a magic marker. The object of the contest is to see who can get the most signatures on their feet in an allotted period of time. Both feet can be used and the signatures have to be legible. The contestants should all start at a different place in the crowd.
"The Wave" is a tradition at many sporting events and is usually fun for everyone involved. It can be started by any group of people and followed by the rest of the crowd. It's done by a number of people standing up, raising their hands above their heads, and sitting back down. From there, the rest of the audience typically follows suit and it creates a "wave" effect among the spectators. Increase the speed every time it gets back to the place it started to make it more fun.