There is lots to do
Some people add a day to climb Ben Nevis. This has the advantage, that it doesn't need a lot of preparation. I'd spend these two extra days munrobagging from Crianlarich onwards. But that's harder navigationwise (and I've been on the Ben, so I don't need to go back). And it needs some planning (which I consider fun too). Browse the munro page of this site as a start. If you are not one of the guys you his happy carrying a big pack in the mountains, it's more sensible to build camp near the or on the WHW and to do the hikes with a daypack.
WanderBert wrote:
Is the GGW to Fort Augustus worth of thinking? Is Glennfinnan a "must see"?
Bert
GGW is great if you like canals and boats.
Glennfinnan is not a must see, but you can walk there from Fort William through Cona Glen. It's the first leg of the Cape Wrath Trail and really nice. There is a regular steam train from Glenfinnan back to Fort William, if you're into these kind of things.