This may be weird but I have gone riding in my bathing suit before. Ppl always think I'm crazy for doing that. But it doesn't hurt my thighs with my leather Saddle so idk if that's just me. I also go riding in short and a tank top.
I wear my jeans and paddock boots honestly. Also either a loose tee or loose tank top is nice:)
If it's reaaallly hot I wear Kerrits riding tights, a tank top, and paddock boots without chaps.
Long sleeve COTTON button up. Coolest option.
long sleeved sunshirt. It has very light material and it will protect you from the sun. Wear light coloured breeches if you have them. The darker they are, the more heat they'll attract.
Tank top! There are also cool tie things that you can wet, and they keep your neck cool. But if it is really really hot and humid than it might be better to skip, you don't want to get heat stroke and pass out or something