Argyll is where Scotland starts to break up into islands and sea lochs. The coastline here is complicated in the best possible way — inlets running deep into the hills, islands of all sizes, ferries threading between them. It takes longer to get around than you'd think from a map, but that's rather the point.
Oban is the main ferry port and a natural base. It's a small, busy harbour town with a good waterfront, several distilleries nearby and some very decent seafood. McCaig's Tower sits above it — a Victorian folly modelled on the Colosseum, never finished, but oddly compelling. From Oban you can get ferries to Mull, Islay, Colonsay, Lismore and further into the Hebrides.
Mull is the most accessible of the islands and busy in summer. The drive across it to Fionnphort, where you catch the short crossing to Iona, is slow but fine. Iona itself is small and worth the trip — the abbey is one of the oldest Christian sites in Britain, and the island has a quietness to it that persists even when it's full of visitors. Staffa, reached by boat tour from Mull or Iona, has Fingal's Cave and the hexagonal basalt columns — it's the kind of place that looks almost unreal in photographs and turns out to be just as good in person.
Islay is the whisky island. Eight distilleries, all accessible, all doing tours. The peaty, coastal style that Islay produces is distinct from most other Scotch, and the distilleries are worth visiting even if you're not a committed enthusiast. The island also has good beaches and significant numbers of overwintering geese.
On the mainland, the Crinan Canal is a nine-mile canal connecting Loch Fyne to the Sound of Jura, hand-operated locks and all. Inveraray on Loch Fyne is a neatly planned town with a castle open to visitors and a decent reason to stop. Castle Stalker north of Oban — a tower house on a small island in a sea loch — is the kind of thing that makes people pull over and reach for a camera. Kilchurn Castle at the eastern end of Loch Awe is similar.
Loch Lomond and the Trossachs are technically on the edge of this area — the loch is the largest in Britain by surface area and well visited, though Balloch at its southern end can be crowded. The Cobbler above Arrochar is a hill with a genuinely distinctive profile and one of the better day walks in the southern Highlands. Luss on the western shore of Loch Lomond is a small village that photographs well and gets busy at weekends.