NIU is an annual thing for me to go to. Aside from food and games to win prizes (money goes to a charity), there's a live show. I actually sat there for a a while to watch people sing and dance. They were great!
My favourite part is of course the food! There's a ton of food and drink stands for you to try. My first thing to go look for would be karage (japanese fried chicken mmm).
Might be a little annoying to find parking though.. so go early!