Hi, we have a holiday house further around the coast. You don’t say if you know the area at all. Southerness, although on the coast is a bit remote, off the A75. Journey further Along the coast and you come to Kipford a sailing village. From there you can do a lovely coastal walk to Rockcliff and back. Lunch in the pub.
I would suggest a trip to Kirkcudbright, the artist town. The Art Gallery ( part of the Scottish National Gallery) is open, as is it’s cafe. Both should be booked. The Museum is also open and bootable. The historic town has a harbour and a castle, lots of interesting individual shops, galleries and two antique shops.
A road trip along the Raiders Road, just North of New Galloway, stopping at the Otters Pool for a picnic.
Threave Gardens a NT property at Castle Douglas is open, and bootable.
September can often be gentle weather due to the influence of the Gulf Stream.
Have a great time.