Vest-Agder in the very south of Norway (Sørlandet) is home to many lovely coastal cities and towns, including Kristiansand, Mandal, and Flekkefjord.
I think this area of Norway is nicest in the summertime and early autumn, when it’s sunny and warm and you can enjoy being outside and even going swimming.
Kristiansand is on the main train line and also has an international airport.
Mandal is Norway’s southernmost town and while it’s not exactly a hidden gem, it’s nowhere near as crowded as the fjords in the summer. The town itself has colorful wooden houses along the water and a cute little center with cobblestone streets, and outside of town you’ll find an 800 meter-long beach, Sjøsanden. If you can brave the cold waters you can go swimming!
It’s about a 40 minute drive from Kristiansand and 2 hours and 45 minutes from Stavanger, so quite easily accessible.
In Mandal I recommend staying at Mones Feriesenter, which has apartments overlooking the sea and offers free canoe and pedal boat rentals so you can enjoy the seaside! Check current rates here