I realize me chiming in here might be kind of odd but I've been up the west coast three times in my life now, the last trip getting deep into the islands.
If you need a night or two away and don't want to drive, just take the train to the Arran ferry. Absolutely lovely and compared to the rest of the west coast ridiculously easy to get to. The Brodick Bunkhouse was minimalistic but swanky design, and the location is quite central.
If you've got a week, it's entirely doable without a car to get to Mallaig and do the four small isles, Knoydart, and the Sleat peninsula of Skye just with ferries. Eigg is amazing on so many levels. Knoydart has phenomenal walking. The Armadale Castle museum on Sleat is the best museum in Scotland that I've been to outside the central belt. Eilean Iarmain hotel is a fantastic setup with a good restaurant and a solid pub. I didn't make it to any of the other three small isles and I think Eigg is the star of the show, but many folks I talked to who'd done them all recommended them as well.
The outer Hebrides are famously hard to get to, and I wouldn't say there's loads to do out there, but the scenery is fantastic. I enjoyed the way-out-of-the-way feel of the hostel at Kershader as much as anywhere, even though the scenery, while lovely, couldn't hold a candle to places like Eigg or Lochinver. North Harris, on the other hand, was the most incredible drive I went on. Heavy, non-stop driving rain couldn't reduce it. Just fabulous.
I've mentioned Lochinver and the rest of Assynt elsewhere here. I didn't enjoy Clachtoll as much as graygo -- while I was supposed to stay two nights, the wind froze me the first night and then the midgies came out in the morning, I was out of food, and I needed to use the internet to solve a mobile data problem and I couldn't get a decent connection. And the locals I ran into were particularly unpleasant. I walked away from a night's camping fee and went to the An Cala bunkhouse in Lochinver and was happy as a clam. That almost certainly didn't give Clachtoll a fair shake and was just bad luck but there you are. But Assynt is phenomenal.