Thanks for all the suggestions, you guys.

I've done just about all of the stuff that you guys suggested last winter at my old barn in the indoor arena, when she had the exact same problem. That's how I ended up dancing around her with a giant orange traffic cone on my head when she decided that they were horse-eating monsters, and how she eventually let me put it on her head. There's not a lot to work with in the current arena, but I'll be sure to focus on the multiple doors and open viewing area. Why is it that a viewing area with glass isn't the least bit scary, but the one without the glass a horse-eating monster?

The indoor arena turnout is hard - I've been doing it as much as I can (bout 1x per week if we're lucky), but the timing/other factors really make it hard. She's also one of those horses who go slightly nuts without turnout; her old owner used her as a lesson & show horse for little ones when she was out 24/7. Pretty different now. Thankfully, I did manage to find another barn here that does winter turnout for the next school year, so we probably won't have the issue next year.

I'll definitely try the singing; the main problem really isn't her. It's me. I'm naturally a timid rider, one heck of a lot better than I used to be, but still get tense on occasion. I can handle whatever the horse actually does without a problem - it's the anticipation that gets to me. And having a horse who notices everything, whether or not she spooks at it, can trigger that.

Last time I rode, I kept her moving and busy the whole time; worked like a charm. Only one spook. I think I also managed to distract myself as well, which is probably a major reason why it worked.