Here's some views from the areas about Plockton, Applecross and the leg up to Kinlochewe from this year.
Plockton.
Lochcarron.
Views from the road.
Here's a few views from a trip last year.
Top of the 'Pass of the Cattle' heading to Applecross.
Personally I'd forget trying to go all the way north to Tongue until you have more time as it will just become a drag on the roads and you will see nothing. To do the north part of Scotland justice and enjoy it you need to follow the west, north and east coastlines around and you will see fantastic scenery but you need to spend 4/5 days. Fuel was mentioned above and it is an issue, there's not a lot of filling stations further north, some may only be a couple of pumps, finding them can be difficult and on Sundays its almost certain they will not be open.
If you are based at Invergarry this time why not go to Skye or do the roads in the above photos following the coast round from Kyle of Lochalsh, Plockton, Strathcarron, Lochcarron, Applecross, Fearnmore, Shieldaig, Torridon, Kinlochewe, Achnasheen from there you can do a view routes which will start to take you back down. The road on the east side of Loch Ness is a cracking road as well if you end up heading Inverness direction.
Just a few thoughts.
If you want to see some views on Skye etc click below.
Skye & West Coast 05
Skye & West Coast 06