Hi, and welcome.

Wearing one of the well-known athletic brands might make you feel like less of a show-off but I think that's more for you than other people. People cant tell what brand something is unless they are looking really close. Probably the smallest brief marketed for athletic use is the sporti euro. Or you can just wear anything in black and it gives it a purposeful, athletic-wear look regardless of what it is. You should feel free to post photos for reference, I will try to include a photo of sporti euro if I can find; I think it would be a perfect pick for what you want and quite inexpensive as well.