KOK floating sauna in front of the Operahouse on a sunny day is a must-do when in Oslo.
Tip: Book in advance and if you have time do the small fjord cruise (the sauna sails out in the fjord).
Dyna Lighthouse is a unique perle in the middle of Oslo fjord just 25 minutes from the Honnørbrygge. It mainly opens for events or such, however a few times a year you can book for a seafood dinner evening with just 8 people.
It is one of the most amazing experiences I ever had in Oslo for sure.
Tip: sign up for their newsletter and get the invitation for the intimate evenings they some times host.