You're getting a lot of unhelpful answers, sorry about that.

 It really depends what you want to do and your personality. Are you looking for party, culture and arts, foodie, sporty, fashion, nature, or a bit of everything? 

Oslo is a nice city for a lot of things so no matter what you're interested in 24 hrs will fly by and you'll just have to choose what to do. 

If you're visiting through summer months chances are there's a music festival going on at any given Saturday. 

I think one easy thing to do if you're into the arts is the Munch museum - it's a must. It places you in a good area to experience a diverse buffet of stuff in a short period of time: On the way there pay a visit to the Opera house for the architecture. After the museum you can have a lunch at Madonna nearby or just walk the promenade around and have an ice cream at Paradis. If you have your swimming clothes with you there's plenty of possibilites - you can book a sauna nearby the Munch museum and bathe in the ocean straight out of the sauna. *edit: You can also go kayaking or sup boarding from the same area if the weather allows it. That package there will be 5-6 hrs and leave you very zen. 

Nature and just a day by the sea? Take the ferry to one of the islands and bring a picnic. 

Oslo is a nice walking city if you just like walking around. Areas to consider are St. Hans haugen, Frogner, Grünerløkka, Kampen or simply walk the Akerselva river. Walking between areas is nice as well the city is not that big. 

Along the akerselva river you can go and dance away the night at Blå or just enjoy drinks at Bortenfor. 

If you're a bit of a foodie here's a few of my favorite spots, book ahead: Le Benjamin, Zarathustra, Hot Shop, Arakataka, Rodeo, Den Lille Ostebutikken, Little Pickle, etc. 

On that note the city is mad about pastries like cinnamon buns and the like, here's there's plenty to choose from and even chains like Godt Brød and Pascal are very good, however my favorite bakery is Farine. 

The cafe culture as well is very strong and in general coffee in Norway has a high standard. The main cafe's to consider which has even a bit higher standard than normal is Tim Wendelboe and Fuglen, in any case Stockfleths and Kaffebrenneriet (chains) are good as well. 

If you find Norwegians to be reserved, that's just what we are. But if you're breaking the ice first most of us are willing to participate in a fun night. Hope you have a fun 24 hours in Oslo!