Beautiful place to come watch the sunset after you...

Beautiful place to come watch the sunset after you've been at a museum (Pushwagner, Astrup Fearnley...) or just sight seeing (a guided boat tour perhaps?). You can get gelato at Paradis, take away food (ie sushi, burgers...) or coffee at Wayne's, or fruit at the Joker and enjoy it on the seating areas.
Open 24 hours, but don't make noise after 23:00 as it is a residential area. You have several options for "city swimming" here too.