Towel pullups (drape a towel over the pullup bar; grasp an end in each hand), see-saw towel pullups. If you have a rope but can't pull your weight, tie a dumbell or kettlebell to one end and pull the rope to you, hand over hand. You can do this outside, along the ground, or you can throw the rope over the pullup bar and hoist the weight to the top. You'll want to mimic the arm motion of climbing as much as possible.