I too received my 85th anniversary QSL card.
My last QSL from Radio Prague had a schedule and
the letter opener thing enclosed, so this time I was
thrilled when I found ... a sponge.

lol, yes a sponge... dry and smashed flat to about
1/8th inch thick... one side has the Radio Praha logo
and "Come ride the airwaves" in many languages.
The other side says I will have the perfect sponge if I place
it in water. I've put it with my other souvenirs from SW
stations.