Where to go, what not to miss? 12 days in May

My wife and I, from the U.S.A. first visit to Scotland, have 12 days to wander around Scotland in May. I am open to suggestions as to a mini itinerary. We keep realizing that all the places we want to bird are too numerous. We have already eliminated the far North. So Caringorms, Isle of Skye, Mull, N, and S. Uist? How much can we do and not feel like we are just racing around? Thank you