Sucks that you can't do regular squats, but it's an opportunity to hit legs in (perhaps) new ways:
Sumo deadlifts - hits legs harder than convential, and the focus is on strength
Romanian deadlifts for hamstrings
Front or Zercher squats for overall legs (as long as you go deep to hit posterior). Must clean for the former, and deadlift, then creatively place in crux of arms for the latter (crouch down while placing barbell on quads, then hook with arms in supinated position - a bit awkward).
Jefferson squats (I think Phil Heath has a video of him doing this somewhere)
Hack squats (not a big fan of this, as it seems unnatural, and seems to place a lot of stress on knees, but I don't really know since I only tried this once or twice)