With its beautifully designed glass tunnel stretching out to the Ounasjoki, Arktikum is one of Finland's best museums, albeit with a hefty admission fee. Exhibition spaces include superb static and interactive displays focusing on Arctic flora and fauna, as well as on the peoples of Arctic Europe, Asia and North America. It tells the story of the people of the North.
The level of information is very impressive; this is really a place to learn about the unique northern environments, and there is an excellent research library. There are also good displays of canoes, dwellings, fishing materials, and costumes of various northern peoples (including a good exhibition on the Sámi), as well as a room devoted to the history of Rovaniemi itself. A scale model shows the destruction wrought by the Axis retreat in 1944. There's also a multi-vision theater and a good restaurant. You should allow yourself at least a couple of hours to get around it all.
The museum is easy to find, located on the banks of the Ounasjoki River and at the crossroads of the two highways leading north of out Rovaniemi. Tickets cost €12 for adults and €5 for school children age 7 to 15.