Teufelsberg is an iconic Berlin landmark that is located just west of the city center, in the Grunewald forest. It's an abandoned Cold War listening station that was built atop a rubble mountain created from the debris of World War II. Access to the site is highly restricted and visitors need to join a guided tour to gain entrance. Once you're on top, you're rewarded with awesome views and a look at ruins of a former listening station. There is plenty of nature and urban exploration to be enjoyed, making this an interesting day trip. It's definitely one of the most unique destinations you can experience in Berlin.