Another super simple one that has always worked pretty good for me on squirrel/rabbit is to marinade overnight in Kraft zesty italian dressing. Season the way you like it before grilling.
Never did this for rabbit but I have on a tough old buck. Cut the meat into cubes, marinated it in Italian dressing for 24 hours, and cooked it over coals as a kabob. It came out excellent! Very tasty and tender.
Another option is fresh, crushed pinapple. The acid and enzymes in the pinapple tenderize the meat. Put the meat and crushed pinnaple in a ziplock in the fridge overnight. Canned pinapple won't work though. The enzymes are cooked out in the canning process.