Not much activity on I2P but there's plenty of eepsites indexed on notbob.i2p under the 'forum' category tho most of them are just mirrors for clearnet sites. You can also try searching with i2psearch.i2p (http://d4jtslmsku66nmq7u3mpyhqb3cw3operuhldvegkectmakmf6jiq.b32.i2p). Out of the more peculiar stuff there is a minecraft server that's been running on i2p for quite a while now called i2pcraft.i2p (http://pkhcrpvyickdaobvmmx2knjixl5a2ckrpavixvxexhurc3n7uemq.b32.i2p)