With my friends we created quite a managable challenge- we're sketching our characters using poses from our photos with friends & family as references. Each of us first selected a bunch of interesting photos (so something with silly poses, or multiple characters interacting, or poses we want to practice - like sitting or walking up the stairs), and now we use random number generator to select 3-5 for each week. It's fun, it's just sketches so it doesn't take much time, and it's challenging (as poses are sometimes hard, plus there are backgrounds, plus we have to adapt what's going on in the photo to our characters' world and settings), I highly recommend