Clear sunny days and equivalently lit environments. Anything brighter than cloudy is too bright, it's like all of that brightness fills my bandwidth and I can't think properly.

Ambient heat above 25°C is another one, that feeling of bulgy, sweaty, tight, sticky meat suit does not help my composure. It's acceptable in some very specific contexts, but VERY specific.

Crowds get my heart pumping as well, too many people to track, variable overload.

On the flipside, I've seen some comments mentioning clutter as being another potential trigger, but I've always found visual noise to be soothing. My brain enjoys picking out the details as a background process, and the more varied/colourful, the better! I still have my limits, though, like I have no problem with messiness, like clothes thrown about the place and tools/resources left on random surfaces, but trash is trash.