Yeah, what
Chief Captain said. I've been instructing for the last year and a half, and the closest I ever came to burnout was when I had nothing but private student, who were all in the same phase (most even the same lessons). As soon as I got some instrument students, things got a lot better, and it wasn't long after that before I got multi students. Just mixing it up really helps. Other things that I like to do is challenge my students to landing competitions, cross country flights where we grab a $100 hamburger (never from a fast food place, ask the FBO folks for the best diner/dive in town), and my personal favorite, messing with the students working on their initial CFI, as well as the new hire CFIs. One of my favorites is to take my broken piece of tie down chain, and place it on a new-hires tail right after they've shut down, then hide. Worth it every time!
Also the CFIs in my office tend to do things for ourselves since we know our school really doesn't "care" about us. We throw birthday, new job, and holiday parties all the time. We have one CFI that loves decorating our office for every holiday, and she really gets into it. You just have to take advantage of the little things, and make it fun. One thing I started doing, is keeping tabs with the new-hire CFIs, and right after they've cut the shirt on their very first student solo sign-off, we grab them and cut off their right sock (for all the pacing they were doing while their student was soloing, and for all the right rudder that was hopefully added without them saying it). By the way, we have an awesome collection of nasty sock pieces!
Some of the things we are trying to plan for the summer when the weather is better, is organizing a flight of multiple aircraft for fly-in breakfasts, as well as trying to organize a trip to OKC to go to the pressure chamber with a bunch of students. You just have to get creative with things, and have a fun loving mind with it. Also having some time spent away from the airport really helps. Being down there 12-15 hours a day, 6-7 days a week, for months on end does wear you down. Take some time to enjoy life. Unfortunately right now, we are having way too much time off because of weather, so we are excited about any chance we get to actually fly. Remember, just bring the fun back into the airport and the job, and burnout becomes a thing of the past!