OK, let's see.
Fishing - the Nith is a salmon river (not sure about the "season") and permits are available from the local council - see
http://www.rivernithfishings.co.uk
Towns - Castle Douglas is nice, as are Dalbeattie, Kircudbright and Moffat.
Historic - Bruce's Stone, in the Galloway Forest Park; Loads of Robert Burns places, but for an overview visit the Robert Burns Centre, on the west bank of the river; Cairnholy, at Creetown, the remains of 2 chambered cairns, from 2000 BC; Torhouse Stone Cicle, near Wigtown - a Bronze Age circle.
Quirky - try the Devil's Porridge exhibition at Gretna (not Gretna Green!) - see
http://www.devilsporridge.co.uk
Bladnoch Distillery visitors centre, near Wigtown
Sulwath Brewery, Castle Douglas, does a daily tour at 1 pm
Visit Wanlockhead, Scotland's highest village, and pan for gold.
Shopping - The Old Blacksmiths Centre at Gretna Green; Mill outlets in Moffat;
Child friendly: Monreith Animal World, at Newton Stewart; Dalton Potterty Art Cafe, at Dalton, near Lockerbie - paint a pot!; Mabie Farm Park, see
http://www.mabiefarm.co.uk ; Galloway Deer Range, off the A712 near Castle Douglas.
In addition there are lots of churches, abbies, etc., the best known being Sweetheart Abbey at New Abbey.
If I can be of any more help, just ask
Cheers
Iain