South of Ayr on the A719 stands the ruins of Dunure castle with the lovely old port nearby as well as some 30s (I think) houses which are a bit of a giggle.
A bit further south still on this road is Culzean Castle (National Trust for Scotland), a lovely Adam castle, with extensive and beautiful gardens as well as quite a good cafe. You get wonderful views right over to Aran and the Mull of Kintyre. Entrance is quite expensive if you are not a member.
Girvan has a museum and art gallery right in the centre and an old harbour. Apparently there is still fishing for basking sharks from here. And there is a boatyard where trawlers are built. The RNLI have a post here, which has been known to rescue on of the local windsurfers who got into trouble while sailing round Ailsa Craig, the volcanic island very evident to the West.
Lendalfoot has a caravan site and some interesting ruined castles to explore.
Ballantrae has a lovely Renaissance tomb in the churchyard, where the main road curves round in the centre of the village. Unfortunately you can only view it through a grill. The formation of a stretch of fresh water behind the stony shore is a protected bird nesting site. The ruins of Stincher castle can be seen just as you leave the village from the south. On the hill above are the remains of a windmill. Knockdolian, the hill inland has a hill fort on a naturally defended outcrop to the North.
To the North of Ballantrae at the top of a steep hill, before you reach Lendalfoot is a car park. From here you can scramble down on a path to the North side to the cave in which the infamous Sawney Bean (Sandy Bain) was supposed to live. Rumour has it that he captured travellers and preserved their flesh in brine for later consumption.
All the way down the coast road, the A77, and also further inland, there are the ruins of about 20 castles between Girvan and Stranraer.
Glen Luce Abbey (National Trust for Scotland) just north of A75 at Glen Luce is worth a visit.