Used to live there, but I'm not in Norway these days. There are some pals that do, they'll probably hook you up. If not, you'll probably run into them anyway, since the scene is small and tight.
There is a small new park at a place called Mølleparken, it's at Sannergata/Waldemar Thranesgate by Akerselva (the river). I think that's where most people meet up these days. There is also a large patch of grass, a small soccer field and a basketball court. You can buy beers and BBQ at the Kiwi right next to it.
City hall (Rådhuset) is also pretty chill and there is usually someone skating there.
Some cool ledges by the Opera (Havnelageret)
You should also check out the islands if you like to swim. Boats go from Rådhuset to 4 or 5 different islands. Hovedøya is the biggest one and fastest to get to.. It's really nice to hang out there in the evening as well. I think the suns up until ~22/23.
For hiking, you have Nordmarka and Østmarka. Basically there is tons of marked and unmarked hiking trails.. I would usually walk from home and up to Trollvannstua (they have alright food, and you can swim in the lake) and just head north/east for a couple of hours (if you end up at Lilloseter they have the best cinnamon buns in the world).. I never had a map and there is limited 3G coverage in the woods, so if your internal compass sucks, then get a map or stick to the marked trails.