As architecture buffs you'll find Oslo worth a couple of days, IMHO. The first thing you'll notice coming into town is the new opera house by Sno/hetta, nestled into the fjord beyond the train station. The Radhuset (town hall, completed in 1950 and site of the Nobel Peace Prize ceremonies), is fascinating in its Art Deco-eclecticism (brush up on your Edda myths to appreciate the bas reliefs in the courtyard). Tourist magnet Aker Brygge, the harbor-front mall opposite the old castle Akhershus, is like a Rouse Corporation project on steroids (and has a very good if pricey seafood restaurant, Lofoten). A trip [ed.: this is not a major day trip, only a 20-minute bus ride, but there's a lot to see] to the Bygdoy peninsula will take the better part of a day but is worth it to see the Folkmuseum, with its rural buildings from all over the country, including an 800-year-old stave church from Gol. While you're in the neighborhood there's also the Kon Tiki, Viking, and Fram museums.

BTW, with reference to your age, you'll feel like youngsters if you go xc skiing--you're likely to see many white-haired couples whizzing past you on the trails. Not sure about ski conditions in March around Lillehammer. In Telemark where I ski that would be a little late in the season, but conditions do vary from year to year.

You don't want to go to Roros?