1. Vigeland Park: One of Oslo's hidden gems, Frogner Park is home to the world's largest sculpture park made by a single artist, Gustav Vigeland. The park features over 200 sculptures in bronze, granite, and wrought iron, attracting both tourists and locals year-round. 2. The Oslo Opera House: Opened in 2008, the Oslo Opera House is renowned for its striking modern architecture and is designed to resemble a glacier rising from the fjord. Visitors can walk on its roof for panoramic views of the city and the surrounding waters. 3. Akershus Fortress: This medieval castle and fortress, located in the heart of Oslo, dates back to the late 13th century. It has served various purposes over the years, including a royal residence and a military base, and now offers guided tours and stunning views of the Oslofjord.