Climb Ben Nevis - the highest mountain in Britain. The summit is 4406 feet high with some dangerous sections very near the path to the summit. It is essential that a map and compass are carried as well as warm clothing and waterproofs. A leaflet, "Navigation on Ben Nevis", is available free in Fort William. (http://www.mountaineering...)
the Mamores
Explore the Mamores if you like your mountains less populated than Ben Nevis. This is a beautiful range between Glen Nevis to the north and Kinlochleven to the south. They include a number of Munroes so you may well meet people but you may well not want to do the Ben Nevis tourist route more than once.
skiing and snowboarding
In winter, skiing and snowboarding at the nearby Nevis Range (http://www.nevisrange.co....), with a gondola and chairlifts up the hill.
mountain bike courses
In summer, there are cross country and downhill mountain bike courses at Nevis Range. The UCI mountain bike World Cup has been hosted here several times. (http://www.fortwilliamwor...)
From March to October one can start the Great Glen Way walk to Inverness from Fort William.