Over the last 7 months, I've learned approximately 100 rock songs on the bass. Most of it presents little or no challenge to play, but it's fun to groove these things with the band. Tom Petty's "Refugee," for example, is an easy song to play, but if it doesn't groove, you've got a problem.
What are you working on? Is there any bassist whose style or particular song(s) you've been learning or practicing?