If you are planning to base yourselves in Glasgow, pick up some brochures from the tourist office in George Square - there are a number of smaller day tours that go out of Glasgow and into the highlands.

Edinburgh makes a great day trip, as does Stirling. If you are looking at renting a car at all, a nice day trip would be to follow Great Western Road out of the city, stopping at Loch Lomond and then heading along to Inveraray. There is some pretty dramatic scenery between Tarbet and Inveraray. OR, at Tarbet you could head in the direction of Crianlarich and continue on to Glencoe. If you don't want to do the driving yourself, some of the day tours mentioned previously do get up to Glencoe.