Variety is something I consider to be important to keep things fresh in order to prevent stagnation (whether physically or mentally). I have been practicing martial arts for a few years now, and it never feels like it is getting old. I am continuously learning new things, refining my techniques, and applying those details to material I learned in the past. In our classes, we tend drill different things that physically affect different parts of our bodies. Practicing martial arts does get easier with time, but I am still finding myself faltering when I learn unfamiliar physical movements. It throws both my mind and body off, and forces me to adapt to what I'm doing.
Setting small goals (while keeping the overall big picture goal in mind) helps keeps things realistically attainable. By breaking things up into little steps, I find myself applying little tweaks to fine-tune my performance so that I can reach the next level of where I want to be. All those small performance jumps contribute to the type of person I want to be at a later point in time. Having a time frame to reach your goal prompts me to work more, but even if I don't complete what I want in that span, the effort I have put in will not have gone to waste. Asking myself who I want to be in 6 months, 1 year, 2 years, or whatever also motivates me to do more. I never want to be stuck on making any progress; the thought of being the same person in the future as now screams, "LOSER!" to me, something I do not want to be.
Looking up to other people has also motivated to become a better performer. I have only been practicing martial arts for a few years, but there are many others who have been practicing far longer than I have. Watching older martial artists serves as an inspiration for me to be better and to challenge and push myself beyond what I can currently do. That itself can manifest in various ways, but it also feeds into the mental effort I put in for tweaking my performance and setting goals for myself.
These are all very intrinsic factors that make exercise personally interesting to me. Conversely, one extrinsic thing that has helped to keep things interesting is to have someone else or a group of people to join you in exercise. They can be friends, random workout people, group classes, teachers, mentors, or whomever. Find people who will push you or train you harder. I like to be around more experienced people who can help provide guidance and insight to how I can strive for excellence in what I'm doing. Without that type of support system and the kind of person I am, I wouldn't be in the state I am today.
After years of martial arts training, I am also finding myself to be very capable in other physical activities. Going back to variety, I am starting to experiment with other things like running (for the Warrior Dash) and rowing (particularly Dragon Boat) and have an interest in pursuing climbing and snowboarding. The physical development I have gained is making me more aware and conscious of what my body is doing, and I am eager to try new things and test myself out in different capacities. You can get a feel of what you would like to do from the ample amount of opportunities available to you.
Aside from that, when I used to "run" many years ago (being terribly out of shape and not knowing what I was doing), I would pretend to chase people around the track and then run away from them when I passed them. Sometimes I would count in my head to see what pace I was keeping; other times I just liked to make imaginary stories up about other people at the gym. As an very introverted person, it put away any self-conscious issues I had regarding exercising in public.