Or you could come off the ferry and go explore the Cairns at Camster a bit south (which I always think are pretty impressive, no more than 30 mins drive from the ferry) or something that way - there are options :-), then wiggle on down the A9.

Another hour or so will get you to Carn Liath, which is smack by the A9 and has some lovely views, nice stop for a cuppa out of the flask and a kitkat down inside ;-).

It's then only 15-20mins from there to the Mound and the Lairg road. I've for the first time visited the tiny stone circle, Aberscross, there yesterday and again, right by the road, not going to delay you more than a 20 mins. You could stop in Lairg for lunch, or take the back road out of Lairg to Falls of Shin for food (touristy place), then head across the country on the Strath Oykel road - you'll want to stop a few times to take pics as you go, but I don't know of any stones that way I don't think... as you go West the mountains begin to rise up before - I go that way at least once a month most of the year and I *never* tire of it (not quite as amazing at the North / North West tip route, but pretty awesome)!

Once you get to Ledmore Junction you're maybe half an hour from Ullapool. If you find by then you have time to kill take a quick jog up Knockan Craig (on maps, theres loos and a big car park by the road, alow I dunno, an hour for the walk) - the views up there even on a gotty day are pretty awesome :-).

If you go the Tongue way there's not so much in the way old stones and stuff, it's majestic - but lots of the same thing really. Going the A9 you've got more choice on what to do your limited stops at - and you still get the WOW heading into the mountains a bit later on :-)