Welsh route

Welsh route ("three castles" + mountains)

Take M5 (or M32) to M4 then exit #21 to M48 to Chepstow,
Right after the bridge take exit #2 to A466 (Chepstow/Monmouth)

Optional: stop en-route in Chepstow to see the Chepstow castle (nothing else to see there)

Continue on A466 in the direction Monmouth till Tintern.

STOP: see the Tintern Abbey ( http://www.castlewales.com/tintern.html )
Also visit Abbey Mill ( http://www.tintern.org.uk/abbymill.htm ) about 5 min walk from the Abbey

Continue on A466 about 10 miles towards Monmouth along the Wye valley.
Before entering Monmouth, take LEFT on A40, direction Abergavenny, do not stop at Monmouth.

Next visit RAGLAN CASTLE (http://www.castlewales.com/raglan.html)
which is about 8 miles west from this junction along the eastwards lane of A40;

It is a bit tricky when you go West, you actually have to go beyond the castle, and then take a U-turn at roundabout to go back towards Monmouth. Similarly, you can exit this castle only in the eastwards direction; so you have to take another U-turn and go back in the direction Abergavenny. On the other hand, you waste only a few miles these way. Otherwise, visit the castle when you backwards (then it's easy) but keep in mind that the Castle closes around 5pm!

Keep going on A40 towards Abergavenny 9 more miles West.

Next STOP: Abergavenny ( http://www.abergavenny.org.uk/ ) , follow the direction for the castle. There's a huge parking nearby. Visit the Castle (well, what remains from it, but it's still nice and gratis!) and the central Market / town hall.

After Abergavenny, keep going on A40 direction BRECON.

All road will be from now essentially a "scenic route", since you have entered
Brecon Beacons national park ( http://www.breconbeacons.org/ )

Do not go all the way till Brecon, but about 1 mile after BWLCH (nota bene: it is not B4560 !), at Llansantffraed, take a sharp left on the unmarked road, direction Talybont-on-Usk.

At T-junction turn right and go a bit on B4558 North-West, pass Talybont-on-Usk, and right after the middle of the village take a sharp left over the canal, (direction Aber village), and after that just keep straight on this road (ignore Aber village)

To the left of you there will be eventually a very long lake, like loch in Scotland
You can stop and walk there a bit, but if you just go on a bit, about 5 miles form Talybont-on-Usk, when you turned left,
you cross a very small bridge.

Stop right after the bridge (there's a free parking to the right), and walk along the stream upward, perpendicular to the road. You can walk as far as you want, and you will see rapids and waterfalls
( http://www.brecon-beacons.com/henrhyd-waterfall.htm , http://jonathancarr.crosswinds.net/Egallery/wales/wales5.html ) one of them is very big (10 metres?) about 15 min walk from the parking

This is the FINAL point of your trip; now go backwards the same way you came,

Except: you can shorten you trip back by taking exit to A449 (direction Newport) from A40, about 9 miles after you pass Abergavenny. It will bring you very fast to M4, and M4 East will bring you home!