I'm not sure if I'd consider them all overrated underrated, but I've a lot of love for the north east coast run.
Amble, Beadnell, Seahouses, Bamburgh etc, and the surrounding areas of course. I'd happily retire to Bamburgh given the chance, and I spent many a year travelling around the other areas doing a spot of fishing and hiking. Going out on the boats to see the Farne Islands/Lindisfarne too, great for wildlife lovers and there's plenty of history on top, I'm tempted to spend a week or two up that way this summer tbh.
Those aside, Jedburgh is a lovely little town/village that I'd recommend to anyone passing through the area.
Edit: typo lol