Balnakeil beach at Durness is worth the stroll out along the sand dunes. If you like post-apocalypse 'chic' then the overpriced chocolate shop housed in a ex-MOD building might appeal.
The Shorehouse on Loch Tarbert, where you catch the ferry to Handa Island, serves lovely fresh seafood.
Stop for some local produce at Drumbeg Stores.
On your way out of Ullapool, stop at Corrieshalloch Gorge. The Inverpolly Road - not the official NC500 route is far nicer, plus there's less traffic.
On the way to Oban, a nice diversion would be to head down from Glenfinnan to Loch Sheil and out to Ardnamurchan Point lighthouse, ferry from Kilchoan to Tobermory - around the west side via Dervaig and Calgary and then ferry from Craignure into Oban. Make sure you book Craignure-Oban ferry as it's very busy.