If you're traveling up Loch Lomond side to get to Fort William, then stop at the Drovers Inn for lunch, or at least a drink (its a must!!) I recommend the mince and tatties. It’s a 100 odd year old pub with some delights inside. Make sure and explore and have a good look round. It’s pretty much the top of the loch on the west side
You could also climb Conic Hill (piss easy) on the east side near the bottom of the loch. It won’t take you long (an hour tops) and it gives great views of the loch from the top.
As for Fort William. Have you got anywhere booked for accommodation yet? I could recommend a few places including MacBagpackers for a Hostel and Glen Nevis Camp site. Every time I go to FW I go to ride trials, so I don’t know what else you could do up there but its got the status of outdoor capitol of the Uk so im sure you could find something to do.
There’s loads of pubs and all are friendly, so don’t be scared to walk down any wee alley ways to get to one... The Grog and Gruel is pretty good for food and beer selection. There's always a bit of night life at the weekend with live bands playing in the Ben Nevis bar, so don’t be surprised how busy it gets for a small town.
Could go on but that'll do you for now. Just ask if you have any specific questions.
ps. im f**king jealous you're riding the Nevis range on the mtbs. Ive been told the red route down from the gondola is immense!