Complex sentence: Heerenveen is famous for its sports accommodations, including the soccer stadium and the Thialf speed skating arena which was one of the first indoor 400m ice rinks in the world, and annually held international events draw large crowds.
Split into simpler sentences: Heerenveen is famous for its sports accommodations, including the soccer stadium and the Thialf speed skating arena. Which was one of the first indoor 400m ice rinks in the world, and annually held international events draw large crowds.