Watch T.V. and make up riffs/mood music to movies, commercials, and whatever. You can also catch onto the bass lines to the theme songs for your favorite movies, shows, and commercials really easy this way too.
Just the other day I was watching "Quarantined" (Good freaky movie by the way) and by the time it was over, my wrist, finger joints, and finger tips were completely fried due to playing so much, and so fast.
Edit:
My favorite theme song to play on Bass so far is the theme to "The Barney Miller Show". My favorite theme song to play on Guitar at the moment is the theme for "King of the Hill".