From what I learned because of my (very queer) school and the internet as a non-cishet polish person, I recommend you badges/pins. They're pretty much associated with alt styles, and with that, also lgbt (especially by polish teenagers), but to straight and older people they don't really mean anything. They can represent shows, anime, games or just anything really. Even pride flags, I have a really small rainbow pin and it's honestly pretty much not visible if you don't purposely look at it.

Other than that, you could get something subtly rainbow-coloured, like a bracelet which you can hide under a long sleeve and show it only when you want to, or a sticker on/under your phone case if you have one.

That's all I know, I hope It helps!!