Check out Summer Sonic Festival
Summer Sonic is a music festival that takes place in August. They have one in Tokyo and one in Osaka. They have a wide variety of music, including Hip Hop, J-Pop, rock and heavy metal. Tickets are 15,500 yen for one day and 28,000 yen for 2 days, so it is expensive. If you have the money, however, it will probably be a once in a lifetime experience.