In addition to Loch Ken there is also Loch Doon & the ruins of Castle Doon (if you look closely you’ll see my first Mavic 60 foot up a pine tree on an inaccessible rocky promontory!). Clatteringshaws Loch has a dam with an outfall, & some great views.
Much of the coastline is worth viewing, and, although it’s a bit of a drive, Portpatrick on the West coast is worth a visit, with it’s old harbour.
As has been mentioned, Caerlaverock Castle is spectacular, & should definitely be on your list.
Plenty to see & do, which is why we’re going back twice this year!