Before you leave Glasgow airport, ask the rental car agent which signs to follow at the roundabouts leaving the airport to get on the motorway headed toward the Erskine bridge. The signs are not self-explanatory and there's more than one roundabout, as I recall. If you find yourself on the M8 headed to Glasgow and Edinburgh, you're going the wrong way.
Along the western shore of Loch Lomond you could stop and take a cruise on the loch. Cruise Loch Lomond operates out of the dock in the scenic village of Luss.
Between Portree and Inverness you'll be driving along the northern shore of Loch Ness. Stop at Urquhart Castle and the Loch Ness Centre & Exhibition. Two very different experiences, both well worth the stop.
East of Inverness are Culloden Battlefield and Cawdor Castle (with a herd of highland coos). You could continue eastward and have lunch in Nairn, enjoy the scenic beach there before returning to Inverness.