Compiling information from several guidebooks, plotting sights on a large map, allowing more time than Google maps says travel will take, and taking everyone's suggestions (thank you for your time and efforts!) into consideration, I have come up with the 2nd draft of our 6 week itinerary.
I won't bore you with the details of that itinerary, but I have 2 more days to play with.
Here are the places (and how many full days of sightseeing we'll have there in parentheses) where I could add a day:
Orkney Islands (3 days), Isle of Skye (3 days), Callandar area (3 days), Melrose area (2 days).
In which 2 places would you recommend adding a day? Thanks very much.

Sort of hard to offer advice in a vacuum -- where are you starting from to get to Orkney and where are you headed after the Trossachs? But with just the info you've provided -- your longes travel time by far is from Orkney to Skye so you probably need to add a stop over somewhere en route between the two. And you could easily add a day between Skye and Callader such as in Glencoe

I was hoping not to confuse the issue with the rest of our itinerary. Sorry if it had the opposite effect.
We are taking the ferry from Aberdeen to Kirkwall.
We aren't going directly from Orkney to Skye, do have a 3 night/2 day stop in between.
We plan to spend part of a day in Glencoe on our way from Aviemore to Callandar. We won't add an overnight there, as DH doesn't want any of what we call "one night stands".
After Callandar, we'll spend 3 nights in Dumfries before we go to Melrose.
I hope that information helps with recommendations of in what 2 spots (Orkney, Skye, Callandar, Melrose) it would be best to add another day.
Thanks!

OK - then maybe a little more clarification. It is hard when you use 'days' . . . does your '3 days on Skye' mean 3 nights or 4 nights? Same with the other spots? If it is three nights then you are only talking 2 useable days, 4 nights = 3 full days, etc.

If you are staying three nights/two days in those places, me personally -- would add a night to both Skye and the Trossachs. There is soooo much to see/do in theose regions.

If however you mean 4 nights/3 days each place -- heck - toss a coin adding days anywhere would be gravy.

Sorry, I guess when I said "how many full days of sightseeing we'll have there ", it was clear to me, but not to others.
Orkney: 4 nights, 3 days
Isle of Skye: 4 nights, 3 days
Callandar: 4 nights, 3 days
Melrose: 3 nights, 2 days
Appreciate your time.