In reply to AdEvo: Good suggestions so far. A slight variation on these would be the so-called Lochaber Traverse over Grey Corries, Aonachs and Ben Nevis. Can do it from Spean Bridge in the north, or ...
Get the train to Corrour in Rannoch Moor, then:
Day 1 - a short day (you've got to get there first)
Past south end of Loch Treig to Lairig Leacach. Camp or bothy.
Day 2
Grey Corries from Stob Coire na Ceannain in the east to Sgurr Choinnich Beag.
Then Aonach Beag (access to this from the col to the east isn't obvious in poor visibility. Some research would be helpful). Then Aonach Mor. Then backtrack slightly to descend to the col at the head of the Allt Daim (this is steep, nasty if you pick the wrong line). Camp.
Day 3
East ridge Carn mor Dearg
CMD Arete
Ben Nevis
Down tourist track to Ft.William
Train home
If the above route (or something like it) was your plan A you'd still have the option of a bad weather plan B along the glens from Corrour to Glen Nevis, and on to Ft.william. That's still very impressive country and would be enough of an adventure in really bad weather (watch out for river crossings if it's in spate)
Here's a route card on the Carn Mor Dearg Arete and Ben Nevis:
http://www.ukhillwalking.com/logbook/r/?i=47
And here's one on the Grey Corries:
http://www.ukhillwalking.com/logbook/r/?i=321
Add the Aonachs to those two and you could piece together the route I've outlined from the map easily enough.
Full description of the Lochaber Traverse also in this guidebook:
http://www.cicerone.co.uk/product/detail.cfm/book/612/title/great-mountain-...