I chose step-ups. I workout at home also so if you got some dumbbells and a box somewhere you might be better with this. Leg extensions mainly hit the teardrop muscle and the goal of 100 reps is to fill your muscles with blood. Step-ups will involve quads/hamstrings and glutes which are all used during the workout.
Takes a little adjusting of height and weight.
Good luck, my workout is similiar and usually I can't even do my special exercise - which is also sissy squats since I'm just so fucked.