I would try all the activities with the friends again, you might have forgotten one, I remember I forgot one before I got to 100%. Also make sure that you've beaten a friend or the computer at pool, bowling, darts, and QUB3D.
Ok I did the QUB3D and it made me go up to 99.50%. So what could I possibly of forgotten or missed that only gives .50%. I redid all the activities, So I am at a loss. Would apperciate any Ideas. Thanks!