What a great question! I had a few tactics that made studying more fun for me. Everyone's different, of course, so I'd very much like to hear the input of others as well.
- I studied in "fun" locations at least a couple of times per week. Usually these were cool libraries (one was a library with a giant aquarium in the front, for example, and another had really comfy couches instead of chairs). Was this an efficient use of time? Not always, since it did take time to plan out where to go and to actually drive there, and since it can be distracting to study in an unfamiliar location. For me, though, it made MCAT prep feel more like an adventure.
- I rewarded myself in non-MCAT-related ways. For example, I'd think of the most delicious thing possible in the morning and plan to eat it at the end of a day of studying. Warning: personal rewards work very differently for different students; some people get distracted at the thought of the reward, and others lose motivation and feel like they need a reward EVERY time they study. But for me, the knowledge that I'd only really enjoy the reward if I felt like I'd worked hard during the day was helpful.
- When I got especially bored or frustrated, I always went back to a topic that I knew I understood REALLY well. This isn't exactly "fun," but it really cheered me up. I liked to walk through the topic in my head, pretending to teach it to a student who didn't understand it. It's a surprisingly good way to feel competent and like studying is actually paying off.