Although there is an awful lot going on in the valley itself... a little outside there are a number of classic sites if you have a car - eg:
http://www.themodernantiquarian.com/site/2756/baroile.html
http://www.themodernantiquarian.com/site/150/kintraw.html
http://www.themodernantiquarian.com/site/669/clach_an_tsagairt.html
And if you happen to approach from Inverary along Loch Fyne there is one of the most exquisite little stone alignments I've ever seen:
http://www.themodernantiquarian.com/site/5714/brainport_bay_solar_alignment.html