For a warmup, I like just taking a light plate 2.5 or 5lb, and doing straight-arm shoulder circles with them, 10 forward, 10 back, rest, repeat a few times. Small circles seem to work a bit better for me than large circles.
Kettlebell halos (or dumbbell) also work well to warm things up, although those usually cross a bit into more of a strengthening exercise since my lightest KB is 35lbs. Not that that’s a bad thing.
