What do you buy a morose Danish philosopher who invented Existentialism for his 200th birthday?
It doesn't really matter anymore, since Soren Aabye Kierkegaard is dead. He died at the young age of 42, already at this time a mostly broken man, an obsessive writer, a lonely bachelor, and a frequent subject of popular ridicule. Like Henry David Thoreau, Herman Melville, Franz Kafka and F. Scott Fitzgerald (with all of whom he shares some sensibility), he died at a low point of literary success, without much reason to expect that later generations would rediscover his work and call him a genius.
But I believe Soren Kierkegaard died a happy man, because he was that rare philosopher who found answers to the hardest questions he asked, answers that satisfied him completely. The questions were of religion, and of how to live a good life, and his answer involved the "leap of faith" or "leap to faith" (a phrase he invented). Kierkegaard was a devoted Christian, but he defied the philosophical norms of his age by expressly refusing to try to justify his belief with reason or logic. The power of religious faith, he pronounced, was in believing without reason or logic.
His belief in Christianity made him a great religious writer. What made him a great Existential writer was the implicit principle that underlies his argument for religious faith: the principle that we human beings regularly think, live and make decisions without reason or logic.
This was as much a "eureka" moment for Western philosophy as Rene Descartes's cogito ergo sum two centuries before. Though Kierkegaard had to struggle to explain his ideas to his bewildered Danish and European intellectual peers during his life, his idea of religion as a leap to faith would spring incredible gardens of original modernist thought: inheritors of Kierkegaard include Fyodor Dostoevsky, Friedrich Nietzsche, William James, Paul Tillich, Ludwig Wittgenstein, Jean-Paul Sartre and many more.