There's a lot of Scotland, and don't underestimate driving times. Argyll is good, but can be rather wet any time of the year. Going around now you should be safe from the midgies. Recommend getting onto some of the islands too, and boat rides are always nice. Kerrera is a lovely little island just by Oban. It has a very atmospheric castle, though the teashop isn't as good as it was so pack a lunch. If you like a dram, make a tour of some distilleries, each has it's own character. Argyll has lots of archeology & standing stones if that's your thing. There are loads of lumpy bits of you like walking too. Pretty much anywhere is good, tourist hotspot are ok, but try to get off the beaten track once in a while.

If you're driving, and going onto country roads, please please please learn to reverse.

If you're lucky you may get to spot some wild haggis, but they're quite rare now. The farmed ones are cute but have a nasty bite so be careful.

Also don't neglect the borders, loads of history.

It may not sound your thing, but consider geocaching... It takes you to some interesting out of the way places.