Getting tired of looking at your arena walls this winter? You can bet your horse is too. Creating different workouts can be a challenge. The first step is to analyze what you're already doing so you can change it up. Aside from changing up your in-saddle habits, consider staying on the ground so you can stay in shape as well.
Work on the longe line not only helps your horse get out some kicks before you get on, but it can also be used to sharpen up transition work. You get to watch what you're used to feeling. Solidify quick responses to halts and moving forward through the gaits.
Work in hand, even if you don't think you know what you're doing. If you have a trusting ground relationship with your horse, you can do it. Think of how you would ask for anything from the saddle. What is the use of your reins? Which leg and what timing? Rotations on the forehand (haunches-out) in circles around you, shoulder-in, leg-yielding--these can all be done in-hand relieving your horse of some monotony if you seldom or never do it.
Work at liberty. Every horse I have worked at in liberty has enjoyed it. There is a multitude of information and educational videos on the internet if you have no idea where to start. It can be a tool for building trust and a way to exercise without any tack at all.
Last, ride bareback. It keeps you warmer and your session will change automatically. You will work on different things and be able to feel your horse better.
Have fun mixing it up. Avoid what my dad calls "too much church work". He has such a unique way of expressing his thoughts!