The Norwegians love skiing, in particular Nordic cross-country skiing, and within a short drive of Oslo, you can get access to almost 1000 miles of tracks through the countryside. In summer, golf is popular, and within an hour of the city centre, there are several courses where you can play a round for around £25 per person.
On Oslofjord, fishing for salmon and other fish is very good, so keen anglers may want to take their rods with them, and buy a local license at a Norwegian post office.
Football is one of the most popular sports in the country, although most of the best players end up plying their trade in the English Premier League, and the standard of local football is quite poor, however tickets to see Oslo's main teams are usually available on the day from the stadium, and are quite reasonable.