The next progression from a walking lunge is to add in a balance challenge. Begin by stepping your left leg forward into a lunge. Push off your back leg and bring it up in front of your body, bending your knee up toward your chest so that you're balancing on your left foot.
Without putting your foot down, hinge forward from your hips and extend your right leg out behind you, reaching your right arm forward (pictured). Return to standing and repeat on the other side. Do 5 on each side.
Training tip: Have a friend or trainer help you get your body as level as possible until you know what it feels like. It can be hard to tell since you can't see yourself.