If you are going to take in Arran think about it as a visit either on the way to Oban or on the way back. You need a night (or preferably two) in order to get around the best of the sites on Arran.
Travel north towards Oban head for Ardrossan => Brodick ferry to Arran (much stoning, overnight stay, more stoning next day) then short ferry Lochranza => Claonaig to Argyll (Oban is one and a half hours drive North of Claonaig via Kilmartin Glen). Do this route in the evening and stop to watch the sun set behind the Paps of Jura from the great stone phallus at Kintraw.
Alternatively reverse this route for a homeward journey.
It could also be done with a very early start from Oban, cross to Arran via Kilmartin Glen, Lochgilphead, Tarbert, Claonaig-Lochranza and do the sites on a loop of the island- possible but a push to complete in a day- and return on the last ferry to Claonaig then back up to Oban.
Arran - visit Druid Auchencar, Machrie Moor, Stronach Wood (RA), Kings Caves, Torrylinn Cairns, Glenashdale Falls (with Giants Graves Chambered Cairn don't forget Torran Loisge 5 mins above), Eas Mor waterfall Kildonan, the massive scale craziness that makes up the Great Stone Heaps of Clauchland (once you realise what it is you are looking at it kinda makes your heart leap), the general weirdness of the stones and chambered cairns at Sannox and the range of massive semi-wrecked chambered cairns hidden in forestry at North Sannox. Lots of single stones around Brodick. That should keep you busy for a day or two.
Suggestions for going through Kilmartin - see everything.
Mull = Iona & Staffa. On Mull itself, do ride the Craignure-Toronsay Death Railway. The Craignure fossil beach. Also go to Duart Castle (not worth paying/going in) you can wander round the grounds for free and re-enact Donald Sutherland's epic scaling of the little grassy cliff below the Castle the location for Eye Of The Needle.
Oban = drive round the coast past Benderloch and take the lovely walk to Castle Stalker. Make a spoof video.
And remember that you are free to roam in Scotland = No Trespass. If stones are in a farmer's field, you are allowed to walk over, see them and hang out.
Enjoy yourself.