Well, I'm not that good a (re-learning) bass player (yet!) so maybe this is too simple? Or perhaps not what you're asking? But I've had some fun in the past by simply reading the melodic notes of a song from a piano or guitar staff and transposing them for bass. For instance with Clapton's "You Look Wonderful Tonight" it's not too difficult to transpose the main melodic notes, and I found it enjoyable fun to play on bass...