Basically most of Wales is scenic but there is a former coalfield area in a narrow band across south Wales which is best avoided - though the A470 expressway between Cardiff & Merthyr shoots you through it in about 25 minutes.
The capital, Cardiff is worth a look and has 4 very different castles within 7 miles of the centre:>
www.cardiffcastle.com; www.museumwales.ac.uk/stfagans;http://cadw.wales.gov.uk/daysout/caerphilly-castle/?lang=en;http://cadw.wales.gov.uk/daysout/castell-coch/?lang=en; www.visitcardiff.com
Pleasant farming country is found to the west of Cardiff. This has http://www.nationaltrust.org.uk/dyffryn-gardens/ and nearby is the St.Lythans Burial Chamber >http://cadw.wales.gov.uk/daysout/stlythansburialchamber/?lang=en
So, you might like to consider staying a night or 2 in Cardiff - or as a day trip from the Brecon area which is about 1 hour away.
However, if you wish to avoid cities altogether and just stick to the country areas, then that to can be easily accomplished. You come up from Cornwall on the M5 and cross the Avonmouth bridge - traffic here is heavy so be careful to follow the correct lane > you need M49 South Wales. You then cross the Severn estuary (M4) on a huge bridge and must pay a toll of £6.40. Just as you approach Newport, leave the M4 at Junction 24 & take the A449 expressway going north. Drive for about 10 miles/ minutes on the expressway and leave at the second junction (Raglan) and take A40 towards Abergavenny. (If you wish to visit the ruins of Raglan Castle - loop back at the first roundabout & then turn L up the driveway. On leaving the Castle - must turn L but then turn L almost immediately and turn the car around to then turn R and get back to route going NW towards Abergavenny. Look at Google Earth to see the road configuration & all will be clear). http://cadw.wales.gov.uk/daysout/raglancastle/?lang=en
Anyway, If you now just stick to the A40, you would have a scenic route through part of the Brecon Beacons National Park. I would be inclined to try and set up a base somewhere in the Crickhowell > Brecon area for maybe 3 or 4 days.http://www.breconbeacons.org
Now, after staying in the Brecon area, you could either head up through the middle of Wales on the A470 to NW Wales or take the A40 westward to Pembrokeshire with detours to http://www.carregcennencastle.com/Castle.htm & http://www.gardenofwales.org.uk and/or http://www.aberglasney.org - though I was not that impressed with the latter. If going to Pembrokeshire, I would not miss out Tenby. Drive to Bosherston - park by church & take walk over lily pools to the coast - turn L & walk to stunning Barafundle Bay.
So, perhaps a second base in Pembrokeshire? Then up the W coast - Aberaeron is worth a quick look.http://www.discoverceredigion.co.uk/english/Pages/default.aspx
The road to Barmouth passes along a lovely estuary. Harlech Castle is worth seeing. The Porthmadog area makes a good base for NW Wales = Snowdonia. http://www.eryri-npa.gov.uk/home .Be sure to visit http://www.portmeirion-village.com ;http://www.festrail.co.uk;
Hope this helps! You might want to try and find the places / routes on Google Earth.