Clava cairns is very do- able as it's just outside Inverness, so is barely going backwards at all, I'd even put it as important as your destination - The wonderful Callanish, I know it's high praise indeed but it certainly stays in your mind as strongly, It's one of those places that the stones don't look that big in photo's but they're big beauties and the cairns are lovely, that's how i see the place - somewhere that stone circles and chambered cairns sit together in perfect harmony, and this is from someone who doesn't really like to have later cairns built inside stone circles [i'm not a big fan of the one inside Callanish itself, and think that the inside of the circle would have been better and as intended without it, but that's all 4000 years ago], just a couple of Months left now....and i've got a feeling the weathers going to be wonderful for you as well, it's a good idea to have a 10 min drive to the Clava cairns to make sure the hire cars going to be alright on your longer journey across that part of Scotland, and like george say's it's mainly just lovely scenery inbetween inverness and ullapool, and doesn't take to long at all really, especially with those long days up there at that time of year, i'm also very Jealous.