Military clothing. Tourists could visit department stores like GUM but were forbidden to purchase anything that was part of an official uniform like hats, boots, medals etc. Additionally, all outgoing baggage was x-rayed because you weren't supposed to take out money or forbidden items, but the people operating the X-ray machines didn't care even though it was perfectly obvious that I and everyone else in my high school group was smuggling out such material.

I wore a red army belt buckle on my jeans for years afterwards. I think it's still sitting in my friend's attic in Amsterdam.