I used to live near Ross and regularly drove down that way. From Ross I'd head to Abergavenny, either across country on the B4521 through Skenfrith (really nice pub there on the bridge next to the castle), or on the A40 south past Monmouth and up through Raglan. Then the A465 Heads of the Valleys road - it's modern and fast, reasonably scenic itself but you can come off at any point and find nice places to visit. Blaenavon is worth a stop. Once you get down past Merthyr you can loop off via the A4109 at Glynneath if you have time, down through Crynant to Swansea. Once through Swansea pretty much everywhere is lovely. I'd not recommend stopping at either Merthyr or Neath, too urban, but anywhere else is nice enough, people friendly, and in places it is like going back 50 years.
