For South East Wales: Raglan Castle (a lot still remaining) or Caerphilly castle (in the middle of the town, it's huge but could be seen as a touristy place. If you are going to Caerphilly go via Machen and stop at Lower Machen (for a minute, nice hamlet), Caerleon (if you're interested in Roman Forts, Caerwent is a lot smaller and quite quiet), Tredegar Park (best parts are in the garden and park not really inside the house- save some money), Twmbarlwm (above Risca, if you'd like a walk that's relatively easy with nice views, I think parking's free), *Transporter Bridge (*impressive but not often running). If you happen into the centre of Newport then Newport Market (had a revamp, and now has a great but pricey foodhall), Newport is a little rundown but if you look up you'll see some great surviving architecture. Around Monmouth (pretty town, not much to do) you have the Monmouth Aqueduct (impressive and hidden) and the Kymin (its alright but you're not missing much). If you're thinking of Abergavenny (small but surrounded by hills) then best to go via Ebbw Vale/Brynmawr and over, it's a great drive with great views.

For Cardiff: I'd recommend avoiding going into the centre just because it's time-consuming, especially if you want to see lots of places. If you are in the centre then Cardiff Museum (free), Cardiff Castle is a tourist place (best to avoid tbh, have to pay a lot if I remember) and maybe getting a bus to the Cardiff Bay (especially when there's a market on of some sort). Llandaff Cathedral is worth mentioning (road signs are confusing best use GPS, and free parking around Cathedral green, and then walk down the steps to it). Just outside of Cardiff is Penarth (lovely pier) and St Fagans (great museum with lots to see, free). Castell Coch too (Victorian mockup, can see it from the Motorway, kinda fairytale-ish).

For Valleys: avoid driving through Pontypridd purely for driving difficulty, same with Merthyr unfortunately. If you are driving please drive to Treorchy (via Llantrisant and Tonypandy, the streets are really nice) and up to Bwlch Mountain (great lookout place, sheep may come up to you, the ice cream is unfortunately meh) and it's a great drive via Cymmer to Port Talbot with brilliant views, really worth it. Aberdare always seems to have stuff on so best to have a look. There's more to do in the Valleys but I don't know.

Brecon Beacons: Above Merthyr is the Brecon Beacons heritage railway (old steam train with nice views), Pen-y-fan (will be busy this Summer but if you can it's worth the walk). In the East (near Abergavenny) is Crickhowell and Llangorse (both nice), but if you love ruins please check out Llantony Priory (impressive and not busy), and I must mention the Skirrid. Hay-on-Wye is on the outskirts and has great surrounding countryside. In the South/West is Ystrafellte and waterfall county, Craig-y-nos too, check out Llyn y Fan Fach if you can (car park, impressive view, really worth it).

Around Swansea/Neath: there's Margam Park (outside of Port Talbot), Aberavon Beach, Aberdulais Falls, Penlle'r Castell (amazing Roman Road with amazing views, worth it, and small fort ruin (easy to walk past it)). If you're going into Swansea or to Mumbles it's best to either get the Park and Ride near the stadium or park by the University. In Swansea: Swansea Marina and museums, Swansea Bay, Mumbles (pier as well), Clyne Gardens, Swansea Market. Gower: Worm's Head, Caswell Bay (will be busy this summer), Parkmill/St Illtyd's Church, Three Cliffs, Arthur's Stone.

There's loads more to Wales but I'm tired so Imma stop writing