I am not very good at all this sight seeing stuff, half an hour should be long enough to see any museum and is too long to see most big houses and castles. There are two exceptions! Blair Castle(the House of Bruar shop is close by too) and Fort George. After that I want to see the wildlife and the scenery, so I am not really a great help. However if you can fit Fort William and the Highland Museum into your itinerary then the Cameron Museum at Achnacarry might be worth a visit.