Pieces of card with doubled-over masking tape on the back.
|Form a short strip of masking tape into a loop with the sticky side out :||Press the loop onto the back of the card :|
The resulting "badge" will ( usually ) stick to clothing, and easily detaches without damage.
Note that masking tape is designed to be removed from a surface after use; other sorts of tape might stick more firmly, but might be less easy to detach cleanly afterwards.
We used the badges in ( at least ) one of our services :