Combine your visit to Oslo with a great biking experience.
A good starting point is Sonsveien station in Østfold, where you can bike through Son and Moss and Jeløya. From here you can take a ferry across the fjord.
On the way from Horten to Åsgårdstrand, you can visit Edvard Munch’s house. Keep going south and you will reach Sandefjord, then Larvik and the small summer town of Stavern.