Hi F - don't agree with Ft William - dull looking town with its back to the loch....good as a gateway to the highlands and Glen Nevis etc, but for one night, first time, not sure could recommend....nice b&bs in town and on the approach though, after Onich and Ballachulish...they may be better options if you want a taste of the outdoors.....
Depends how you are travelling, and from where...what do you want to see? Scenery? the mountains? waterside? get a feel of the towns and people?
If you are to fly in, Aberdeen is unique and offers something a bit different; Inverness too as the real capital of the highlands might be worthwhile, and gives access to Loch Ness of course..; Glasgow - fly into Glasgow or to Prestwick and travel on from there - damn fine shoping, best Victorian city in Britain, lots of culture if you want it, tons to do and certainly Scottish - Sauchihall Street for 10mins will make that clear....if you go to Prestwick, maybe go down the Clyde coast to Ayr - Burns country and all that....; and Edinburgh - World Heritage city, big castly thing to see and visit (takes 2 hrs minimum to do properly) plus an old volcano in the middle of the city too to climb, gentrified, popular - millions come each year to 'do' Edinburgh so must have something going for it - and the festivals in August guarantee a good night. But can be expensive accommodation in August because of the festivals though, so suggest a stay in Glasgow and train through to Edinbrugh perhaps - only 45 mins and trains run avery 30 mins ....then you can see two sides of the country...
The Borders if you are driving up, have places like Jedburgh, Kelso, Galashiels, Moffatt - nice wee towns. Lots of scenery, very laid back.....
Up the left of Loch Lomond are places like Tarbert, Luss, and at the foot of the Loch is Balloch as mentioned above - has the Loch Lomond and the Trossachs Gateway Visitor Centre so use that as a place to get your