1. Fredrikstad Fortress (Fredrikstad Festning)
2. Overview: Just a short drive south of Oslo lies Fredrikstad, home to one of Northern Europe's best-preserved fortress towns, Fredrikstad Festning. Founded in 1567 by King Frederick II of Denmark-Norway, this star-shaped fortress invites visitors to step back in time with its cobblestone streets, historic wooden houses, and charming cafes. The fortress is surrounded by moats and still retains much of its original defensive walls, offering an insight into Norway's military history. Rating: 4.6/5
3. Visit Duration: Approximately 3 hours

4. Afterwards: Enjoy a traditional Norwegian lunch at one of the local restaurants before heading back to Oslo.

5. Hiking in Nordmarka Forest

6. Overview: For nature enthusiasts, Nordmarka Forest is an oasis within easy reach of the city. This vast forest offers numerous trails suitable for all levels of hikers, leading through lush forests, past pristine lakes, and up to panoramic viewpoints. Experience the tranquility of Nordic wilderness, where you might even spot wildlife such as deer and moose. Rating: 4.7/5
7. Duration: A full-day hike, around 6 hours including breaks

8. Highlight: Pack a picnic to enjoy by one of the many lakeside spots.

9. Drøbak and the Oscarsborg Fortress

10. Overview: Take a ferry or drive across the Oslofjord to the picturesque town of Drøbak, known for its charming Christmas House and cozy harbor. From here, visit the historic Oscarsborg Fortress, a coastal fortress that played a crucial role in World War II. Explore the tunnels and learn about its strategic importance while enjoying stunning views of the fjord. Rating: 4.5/5
11. Visit Time: About 4 hours

12. Don't Miss: Try "Kanelboller" (cinnamon rolls) at a local bakery in Drøbak.

13. Viking Ship Museum (Vikingskipshuset på Bygdøy)

14. Overview: While technically in Oslo, the Viking Ship Museum deserves a dedicated day trip due to its historical significance. Located on the Bygdøy peninsula, the museum showcases some of the world's best-preserved Viking ships, along with artifacts that shed light on Viking Age society. Witness the Oseberg, Gokstad, and Tune ships up close, marveling at their intricate carvings and sheer size. Rating: 4.8/5
15. Opening Hours: 10:00 AM – 4:00 PM (varies seasonally)
16. Combine With: Visit the nearby Norwegian Folk Museum for a broader understanding of Norwegian culture and traditions.

Each day trip promises a unique experience, allowing you to fully immerse yourself in Norway's natural splendor and rich cultural tapestry. Plan your excursions wisely to make the most of your time in and around Oslo.