I'd stick to North Wales (amazing mountain scenery, castles till your eyes fallout, lovely gardens, and beautiful sea coast)

But what I'd consider is maybe fly open jaw into London and home from Manchester (or vice versa depending on which end of the trip you plonk the weekend).

There are few if any places in N. Wales that can do a one-way car hire. But you could take the train to Chester, get car and return it at MAN before flying home.

If you can't open jaw for some reason -- you could train or fly up to MAN, rent your car there, drop it back there, and return to London/LHR

for a short trip, I'd opt for south-east wales, in particular the Brecon Beacons.

you could get the train to Cardiff, hire a car, drive up to Brecon, then work your way back to Bristol/bristol parkway, and train back to London, or for a slightly longer trip, drive up to Carmarthen and then do the same thing.

loads to see, lovely scenery, not much traffic.

I,d probably do South Wales/Welsh border area. Train to Cardiff, rent a car and book a farm B&B in the Monmouthshire area. See http://www.farmstay.co.uk/ and search on Wye Valley and Valley of Usk. You'll get mountain scenery (the Brecon Beacons), castles (Raglan, Chepstow, others), Tintern Abbey, easy walking along the Brecon & Monmouth canal and the Wye Valley Way, or slightly more challenging, climb Sugarloaf at Abergavenny, medieval cathedrals at Brecon and Hereford. And if you decide you want some city life, just head back into Cardiff.

You have some good advice.
I would take the train to Cardiff, it takes less than 2 hrs from Paddington, pick up car near the railway station and head out.

Another nice place is the Neath Valley, lots of wonderful walks along rivers where there are loads of waterfalls.

http://www.npt.gov.uk/default.aspx?page=2810

Check this site out for castles.
http://www.castlewales.com/

Good luck

Muck

We spent a long weekend in Tenby at this lovely B&B and just loved it: http://www.tenbybandb.co.uk/index.php

The hosts are amazing, breakfast fantastic, and we did some driving around the countryside and visited beaches, castles and cathedrals. It was amazing, and definitely not roughing it.

You could pick up the car in Cardiff.

yes -- doing South Wales gives you the opportunity to do a one-way rental. Train to Cardiff, tour around, drop car at LHR.

Thanks for the replies so far. This is really helpful.

We will have to fly in and out of London because I'm attaching this trip to a business trip and can't change my point of departure.

Does anyone have any info on how crowded the coast will be in South Wales in August if we choose that option? I realize that it's August (holiday season) and it won't be possible to avoid tourists, but are we talking Hamptons long weekend bumper-to-bumper (or Blackpool seaside?), or something more manageable?

Also, one other question. I am mosquito/midge-phobic but the little critters love me. How bad is the bite-y bug situation in Wales in early August (for example, in Snowdonia National Park or by the shore)?

thanks