A wee bit late in seeing your post as only just joined and likely you are both already there. Anyway for what it's worth and very much bearing in mind that you stated Cumbria to Loch Lomond was not too far I think a tour of the whole area would be a walk in the park for you both
I would suggest a visit to Loch Ard near village of Aberfoyle. Luss and nearby Lomond Shores for Shopping. Maybe Lake of Monteith for misty fisherman type shots and nearby abbey. If you want castles there is Stirling Castle and Wallace Monument always a treat (very understated comment). Further north Kilchurn Castle and for a complete change of scene I would recommend the jaw dropping scenery of the "Great Glen" but you are leaving the loch Lomond area and heading north toward Fort William and Oban the Gateway to the Isles. Also there is the Isle of Seil which is not an isle but the once home of the late HRH Princess Danna's mother well worth a visit (micro brewery) and again not too far from Fort William. I hope you both have a very pleasant trip and could have added so much more and what I did add is not in geographical order so visit a tourist information office as petrol is expensive the further north you go
