I would recommend doing searches on this board for Scotland, Glasgow & Edinburgh, as there have been a number of posts made over the past 6-12 months.
If you are going to the east coast of the Northern Highlands, I would highly recommend The Quiet Piggy, which is located in the Bridge Hotel in Helmsdale. A modest menu of 3 or 4 starters, 3 or 4 first and 3 or 4 second courses that changes often. The food is well prepared and not fussy. I would also recommend Luigi's in Dornoch, which serves dinner 4 or 5 nights a week. Lunch is nothing special, but they do a nice job at dinner. Again, a modest menu that changes often. The mussels and other shellfish are highly recommended.
I've got a few recommendations for places down the west coast...
Go to the Crannog seafood restaurant in Fort william - the best langoustine I have eaten and the fish we had for main courses was spot on - perfectly cooked food and an interesting menu without being too fussy.
Try Ee-Usk in Oban - great mini fish casserole type starters and all the fish served is top quality - but I was disappointed with their mussels and chips, as they were just served in the "broth" from the mussels (ie. slightly fishy sea water) - not even a shallot or a dash of cream to be seen.
I'm not sure if it's there still, as it was very new when we ate there 2/3 years ago, but also in Oban, it's worth trying the Oasis restaurant - we really enjoyed the food there and we left hoping that the couple running it would do well, as they were working hard to make it a special place to eat.
The Tayvallich Inn on Loch Sween is quite a detour (wherever you're going, as it's really off the beaten track) but worth the trip. It's a modern pub/restaurant which serves lovely fish from the loch as well as meat dishes.
For very fancy food and surroundings, go to Knockinaam Lodge near Port Patrick. Stunning location, amazing food and you'll leave feeling replete and generally very well cared for!
If I remember anymore, I'll post them later. Enjoy it.