Anything to do with snow and ice in winter. Mount Nuolja's ski lift is open almost all year round for downhill skiing and Kungsleden and other trails take you cross country. There are also small hills here and there for tobogganing and the Abisko Canyon in later winter can have ice formations strong enough to climb.
Equipment: The fell station shop has skis (+ boots) and snow shoes to rent in season and they also lend children their toboggans.
The most famous hiking trail in the country starts right here and dwindles its way down the length of the Swedish fells. You can walk for just a few kilometres and return, or you can set out on a proper trek, with or without your own tent. For those without, the STF has mountain huts here and there (see maps on their homepage when you plan) with various amount of service. Some have only a place to sleep and fresh water whilst some have mini markets too. All have a great atmosphere. Be sure to tell people you ARE leaving when walking in the fells and prepare for any kind of weather as it can change quickly. In early summer, the streams along the trail are full of mountain water and can be hard to cross. In early autumn (full autumn up here!), trees are ablaze in colour. In winter you can ski, but then you have to be sure how to cope with bad weather even more of course. Every season has its charm.