Birkelunden is one of the most frequented parks in Oslo. The park is situated in Grünerløkka, a residential area in town mainly consisting of well-maintained apartment buildings from around the 1880s, and newer buildings that are built in the same style. The area is known for its lively nightlife, and there is a wide selection of restaurants and bars.
The park itself is one of my favorite hangouts during summer. It is a big square city park, with lots of benches to sit down, a fountain in the middle and a music pavilion. The park is often used for festivals and different kinds of events.
There is an all-year-round Sunday market in the lower part of the park (every Sunday with few exceptions, check their website for exact dates). It is mainly a market for antique and second-hand items, ranging from kitchen equipment, clothes, and jewelry to furniture, but you will also find some people selling arts and crafts.
If you are up for more handicrafts and selected second-hand items, check out the Sunday market at Blå article.