Agree that York is lovely but I'd head a bit further north than that. I would assume that you're heading up the West Coast to Kyle (not an attractive town at all but a great drive up from Loch Lomond). Think I'd aim for somewhere like Lancaster or thereabouts.
if you fancy a different but similar mileage route back try going east from Spean Bridge to Dalwhinney and south via Pitlochry, Perth and then either Edinburgh and the A1 or Glasgow and the M6.