Nida

The only word of explanation on this lovely card was "Nida", apart from the Lithuanian stamp.  That was enough for Googling though, and I found out that Nida is a resort on the Baltic Sea, on what is called the Curonian Spit, and the Curonian Spit is what you see on the card.  It is a long curved sand dune that separates the Baltic Sea from a lagoon.

The spit is special because the shifting sand dunes are under constant threat from the winds and the sea.  It isn't only sand though.  There are wetlands and forests too, and it is an important stopover point for migratory birds.  It is listed as a World Heritage Site by UNESCO.