Aviemore. Lots of walking, near castles, has a steam railway, RSPB Osprey centre, beach on Loch Morlich! You're also well placed to get quite far within a hour's drive in many directions. Only thing I would say is it's not a gourmet eating place!
I have booked already to go to Mull next summer, primarily because I hear the wildlife viewing is excellent. Can't wait! I'm actually going on my own, since viewing wildlife is a big interest of mine and I won't be able to do much with DH and dogs in tow. I'm a bit nervous about the driving on narrow roads, though (I am actually used to single track roads, as there are quite a few where I live but, god, I hate them!). Oh, and about going on the car ferry - I've never actually been on one, never mind doing it on my own!
LordRothermere you shouldn't hit single track roads till you get to Mull itself, assuming you're getting the ferry from Oban. Once you're there, you won't want to go fast (even if you could); you might miss something! And remember Mull isn't very big. The Calmac boys will guide you into position on the ferry; I'm sure you'll be fine.
I would go to Ardnamurchan Peninsula/Sanna Bay Sanna Bay photos and split my trip, taking a ferry directly to Mull (Tobermory - Kilchoan which is only ten minutes from Ardnamurchan) for half of the time. Maybe going home from the other side of the island.