I've never been, but looking quickly on a map and here on TMA, it looks as if there's plenty about!
Up toward the NE from Douglas, at Laxey is King 'Orry's Grave, a little furthe ron is the Spiral Stone, and further still is Cashtal yn Ard. Those three sites alone seem to have the most posts here on TMA.
How easy the drive is, timewise, I'd have no idea I'm afraid. Doesn't look too bad though if you have most of a day to potter up the A2.