Colonsay is well-worth looking into. An extraordinary island-hopper is available but only a couple of times per week (and only in the Summer I think). The Kennacraig - Islay ferry returns via Oban twice a week calling at Colonsay. Like on a service bus, a determined traveller can manoeuvre themselves between Argyll, Islay, travel past Jura and get dropped off on Colonsay (say for a few days) and then return to Kennacraig via a (non-alighting) stop at Islay on the way back. Worth a bit of careful timetable-reading, planning and careful booking of bunkhouse on Colonsay.
Raasay is very quick, cheap and easy but the wait at the terminal at Sconser can be... errr .... long and exposed. Ten minute crossing. Fantastic wild camping. Get up to Hallaig and look for the Five Sisters. Sorley MacLean's fave place. Just be aware that deer are culled on Raasay and if you are rumbling around in the bracken looking for corn-drying kilns or old illicit stills when it is "in season" it might be a little inconvenient...
As for Lismore. You can get the more expensive Oban - Lismore ferry which takes an hour. Or you can catch the ten minute crossing from Port Appin a little further up the coast (a very beautiful short journey). Lismore is a pleasure to walk (it is almost completely flat) and full of stuff. I was there in October 2016. Wonderful.
Not worth taking a car across to any of these places IMHO. Bikes are available for hire on all of them. All are very visitor friendly.