Not to be pedantic but Mallaig isn't near Aviemore!
I think going on the train from Glasgow Queen Street to Fort William and then on to Mallaig (but crucially stopping at Morar and not bothering with Mallaig, which is a bit uninspiring itself) is one of the loveliest train journeys and is, of course, the Harry Potter line.... Then go for a walk around the shore of Loch Morar, play on the wee beach, walk back and catch the train again, having checked the timetable very carefully. Loch Morar is stunning.
Sanna Point on Ardnamurchan peninsula is another place that makes me go tingly...nice beach too. You can stay in Kilchoan (wee village, has a shop) and watch the weather come in. And say "Ooh, there's a bit of weather over there..." in a very British manner.
THe Orkney Islands are amazing. Very green, very windy, treeless, almost, but chock full of neolithic monuments. There's Skara Brae and the Ring of Brodgar and Maes Howe chambered cairn with Viking graffiti. Stunning beaches. Loads of WW2 history as well, like Scapa Floe and the Churchill Barriers (complete with shipwrecks visible at low tide) and the Italian Chapel, painted by the Italian POWs. And all over, people who have dropped out and come north to do their thing, spinning, weaving, pottery - a sign points you down a dirt track and there's a shed with someone doing something creative. And they have a folk music festival every year if that's your thing.
Oh and the Isle of Mull. Bala Tobermory is almost the least interesting part of that.