Earlier someone suggested that a monument to a prominent atheist would be an example of a monument celebrating atheism. I am not sure if these qualify. I can think of many people who are atheists and reached prominence, but I cannot think of a single one who reached prominence primarily as an atheist. For example, when I see a monument to Marx I see a monument to Marxism, or communism, or dialectic materialism, -- all signifying his belief, however misguided, in
something, as opposed to an absence of belief in God.
There is a swamp in northern Russia called Solovki. It is a site of a death camp where Stalin put all but most docile of Orthodox priests. The camp is now in ruins. Perhaps coils of barbed wire rotting in a swamp would be a good symbol for an atheist monument.
Mattger of fact, I was wrong. Before Solovki became a death camp, it was a monastery. So while that place is a good illustration of atheism put to practice, as a monument it is still a monument to Christian martyrs.
(See Solovki).
Atheism cannot have a monument.