If you time it right with the wee ferry across the Kyle of Durness (if he even runs at this time of year) you could cycle out to Cape Wrath and back. I think its only ~30km approx on easy almost roads, although the diversion to Kearvaig bay is quite good fun (and very worth it for the scenery/birdlife).
You could also cycle up Strath Dionard from Gualain House. This is a very easy landrover track, but the scenery gets pretty impressive as you get to the Loch (if you have time pushing up the western shore of the loch brings you to a really impressive waterfall with a wee scrambly path up the side of it).
Finally, you could drive to Achfary and ride up Bealach Horne from Loch Stack. You get amazing views of Arkle and Fionaven (eventually) and then a pretty long fast descent back down. Again though, there’s no singletrack really, just a relatively well made track.
Edit: Obvious choice maybe, the track from near the end of the Kinlochbervie road (Balchrick) to Sandwood bay. Its all pretty well made and its pretty spectacular when you eventually crest the rise above Sandwood.
