OK. I don't know the area well, but we did a lovely few days starting and ending in Glasgow (Houston, actually) and going via Arran, Kintyre, Bute and Greenock. Other riders we met on the same trip did the same sort of thing but extending via Islay and Jura.
The upside - near Glasgow there are great bike paths; there is little traffic outside of ferry times; the scenery is lovely; it's pretty easy to get to.
The downside - the scenery is lovely and you have to go over it (long hills, but not usually too steep); midges; weather; you're on the only roads so there is some traffic.