Sounds like a fabulous trip @KJH
I'm going to start by stating the obvious and say even if you don't like gin or whisky - a trip to the Harris Distillery really is a must. The tour itself is really interesting and quite possibly one of the best ones that I have been on. Telling a story of their whisky, gin, Harris, their distillers and other staff - it really is special.
Back on the mainland. If you are travelling from Ullapool to Jo'G - I would suggest a scenic route of driving through Sutherland and into Caithness (part of the North Coast 500).
Dramatic cliffs, empty beaches and the mountain scenery are some of the best remote wilderness you will find here in Scotland. Smoo Caves are worth stopping for too.
I would recommend a drive out to Dunnet Head (Most northerly point of UK mainland) to spot puffins and other seabirds at that time of year.
Also Duncansby stacks are very impressive and well worth going to see - not far from John O'Groats. Brilliant coastal walks along here too.
Mary Ann's cottage is also well worth stopping by to see. It is a crofting cottage that was lived in by three generations of the same family. Mary Ann lived there until she was 93 and the cottage has been preserved to show a way of live. Really interesting.
There is a campsite in Thurso which faces out over the sea with nice views and the harbour and Orkney.
One of my favourite places to eat in Thurso is The Pavillion. A really nice seafood/steak restaurant and I would recommend booking as it's quite small but very popular.
One of my favourite castles - Castle of Mey. Really good tours around the castle, once owned by the Queen Mother. The gardens are beautiful and the cafe onsite provides some amazing food with locally sourced produce.