Fisrt take plenty of Midgie spray..the place is absolutley awash with them..especially near the Lochs.
We have been over to the west caost area a few times...great for landscapes.
In my own humble opinion some of the better known lochs were disappointing in as much as you can drive around them but cant see much of them...Loch Lomond and Loch Ness are two that come to mind.
Many of the castles are no more than a pile of rubble.There are some great ones but check out for images of any of them on the net before going there.Plus your going to pay a price to go in.
Loch Sheil is beautiful from the Glenfinnan area..plus there is a huge viaduct there where a steam train goes over from Fort William.Check the times on the net to make sure your at the Viaduct well before it comes.
There is a car park at Glenfinnan ( you have to pay ) but it is only a very short walk to a great vantage point for super shots of the viaduct..train and Loch Sheil.   
The drive from Fort William to Malag is beautiful with several little lakes on the way.
Loch Linnhie is beautiful..on a calm day lovely coast shots..in poor weather lots of very moody shots with clouds hanging over the mountians.
Not sure if this is what you want but tell you more if you need it.
You can see some shots on my site at

http://www.eddie-john-photography.co.uk/scotland-galleries.htm

Eddie