Everyone gave some great ideas! I agree with working on those corners and getting your horse nice and round. In the winter/spring I only have a small space to work in as well. It can be frustrating sometimes, but it really makes you work your horse neatly. Try to perfect the circle, go across the midline (as suggested), do transitions and whoas. You probably won't have much room to canter, but take it as an oppourtunity to perfect the trot, both sitting and rising.