Oslo is blessed with a huge number of lakes that surround it, plus a decent coastline, and to all these nature has added a beautiful river that somewhat cuts the city in two and offers great relaxing places all along.
It’s one of the best places to go for a picnic with friends, play outdoor games, take long or short walks, enjoy the waterfalls (that freeze in the winter and become beautiful sights), feed the birds (but NO bread or sweets please), take amazing photos, run. And once you get bored of all these, you simply walk a few minutes and mainly end up in a region with bars or shops since plenty of these have been built on the streets parallel to the river, especially in Grünerløkka.
The river starts up at the lake Maridalsvannet and flows all the way down to the fjord ending its journey next to the Opera house. The walk along this ‘green lung’ (which can be started at any point and it would still offer enough beauty) gives you the chance to admire old factories, mills and buildings, making it a visual history lesson.
My favorite spot along the river is the Møllerfossene where a huge waterfall forms at ‘the top’ as well as a bunch of tiny ones at the bottom of the big one.