Visited here whilst in Oslo. Lovely vibrant area with shops, restaurants and cafes. Walking along the promenade is lovely, with lots of yachts in the harbour.
We took the B1 ferry from here which is included in a public transport ticket, so you can have a trip of the fjords for nothing. It leaves the harbour here, visits 5 islands (all of which you can stay on as they have great beaches,etc) and takes around an hour. A perfect way to see the fjords.
We then sat on the harbour front with ice cream and watched the world go by.
City Hall is impressive and the gardens at the front are nice.
We also walked up the Akershus Fortress which is two minutes away and is free, with a small museum, public toilets and a lovely park.
I would definitely recommend visiting this area!