I agree that coming outside of July/August time is a good idea to avoid the "worst" of the crowds. That said the atmosphere in Edinburgh during the festival (August) is very special.
I recommend a trip to Skye, hiring a car and driving via loch lomond and up the west coast (assuming you will arrive in Edinburgh/Glasgow). The scenery is fantastic and if you give yourself a few days you will have plenty time to stop off, Scotland is pretty small so it's no problem getting around if you have a car.
The 3 chimneys is a very well thought of restaurant on Skye, unfortunately I had to to cancel my booking so haven't got to try it yet, but friend's have recommended it highly.
I should admit I am an Edinburgh boy, but you should at least give yourself 3 full days there, there is an incredible amount to see.
The witchery is a good bet for a special dinner or restaurant martin wishart, which is probably the best in Scotland. These aren't cheap, mind you!
Another thing you could consider is a trip during the 6 nations rugby tournament at the beginning of spring time, Edinburgh really comes alive and there will be plenty kilts on show