Grab tickets to watch a game at Tokyo Metropolitan Gymnasium. If there's nothing scheduled while you're there, you might like to check if they give a guided tour of the grounds.
You'll have sightseeing choices galore when staying in Shibuya. Tokyo Metropolitan Gymnasium is a good place to kick off here, but there are loads of other worthwhile attractions to appreciate. Tokyo Skytree, Tokyo Tower, and Tokyo Imperial Palace are not far from the sporting facility and and will cap off your day nicely.