I had exactly this problem with my warmblood mare. She was perfect all summer, but in winter she (and I?) hated the indoor, and by month two, she was spooking at the door, the mounting block, the spot on the wall, etc. She was basically bored to death, and inventing games. But I was not a very confident rider then (this was several years ago), and would start thinking, oh god, here comes the darn spot where she spooks. And she had big huge spooks, too. What helped much later (after time off for an injury) were two things - one, full turnout year round. Two, I am in the ring to work, and she is doing what I tell her. We don't wander around until something exciting (spooky) comes along. We are always doing interesting patterns. Shoulder-in down the long side, circle in the corner, come up the 3/4 line, leg yield over, serpentine back down the arena, back through some poles, change direction again, repeat on the other side, etc. I find "mazes" of poles on the ground or cones can be useful because it gives you something to look at, and forces you to be precise in your turns and patterns, which makes you pay more attention. Especially if you can't give her some more free exercise, as others suggested, make up a bunch of patterns to work on (or look online- someone out there has a book called 101 Arena Exercises or similar, I think), so that you are never just doing boring stuff like straight around the track or big circles, and her mind and body are kept very busy. More importantly, YOUR mind is kept very busy, and you don't have time to think about the spooks coming. My mare was also much happier working when another horse was being exercised in the indoor at the same time, although that wasn't always possible.

Ironically, my husband recently started riding that mare, and he is a total beginner and doesn't know about spooking. He just doodles around the ring on a long rein, and she never, ever spooks! So annoying!