We are interested in standing stones and barrows. I know that Lewis is better for stones but is so far away. Can we take the ferry to Lewis without a car and then get a bus to Stornaway? Are there any stones on Sky? We're going in late September/ early October.
Hard to answer questions about transport without knowing where you are starting from. Where would you be taking a ferry from to Lewis? And, by the way, it's Skye (with an E) Here's a link to a website about ancient sites and standing stones on the islands of Scotland.
Closer to "home" in Scotland is Kilmartin Glen, which has stone cairns and circles. It's near Lochgilphead, about an hour out of Inveraray (which, BTW, is a charming town and has a wonderful castle to visit). If you go, stop at the little museum in the village of Kilmartin and pay for the map (I think it may cost 1 pound), otherwise you could drive right past the stone features because they're not well signposted. There is an enclosure outside the church that's full of fascinating carved stones from the 11th-12th centuries onward.
There is also Clava Cairns, east of Inverness and not far from Culloden Battlefield and Cawdor Castle.