North Wales, Midlands, and Stonehenge Area
Last month we completed an even more ambitious journey around South England and Wales, which saw us skirt the entire Devon, Cornwall and Welsh coasts, and then travel right back through the centre of Wales (North to South) in just 10 days. So it can be done, but yes it was tiring and we needed a holiday afterwards.
That said, we were able to form much opinion on places you are considering, since we visited them all.
With regards to Caernarfon vs Conwy... Conwy wins. the castle is beautiful, though as acknowledged involves a certain amount of stair climbing. The town itself is all contained within old town walls though, and for that is beautiful and makes a nice place to stroll.
We spent one night not too far away in a place called Abedaron (http://www.almostswiss.ch/gallery/United-Kingdom/ - Photo gallery of whole journey) where everyone around us in the lovely hotel spoke only Welsh. In fact, communicating in English was occasionally slow. It was a great place to stay, one of the best in our journey. The hotel is right on the beach, the rooms are beautifully decorated, prices good, and they have a lift. (http://www.gwesty-tynewydd.co.uk)
Another great option of a place to stay near to Wales, but back on the direction of London was The Pheasant Inn, near to Chester. A lovely country inn with beautiful rooms and a great restaurant... all over-looking rolling farmland. (http://www.thepheasantinn.co.uk)
We also stayed at Ross-on-Wye for a night. Here we stayed in the Premier Inn (a chain mentioned in the first answer). Premier Inns are budget hotels that are consistently clean, well-priced, located on the edge of town, and with a restaurant attached. We will happily return to them in the future, since they are one of the few places that allow one night stays during high season.
That is something you need to be aware of, and as suggested, book ahead to avoid disappointment.
While in the area, if you like books, you might like to visit the village of Hay-on-Wye, which hosts a lot of amazing second-hand book-stores. Has to be seen to be believed really.
We stayed in one B&B during our trip and regret it. Inns and budget hotels were all cheaper and with better facilities.
In the vicinity of Stonehenge. Visit the Henge (if you book a night at a Premier Inn you can print a voucher for 1 person free) and on your way north toward Wales, visit the village of Lacock where there is a very nice abbey (easily accessable, flat entrances etc).
Also, just to the north is Castle Coombe, which lives up to it's reputation as being one of the most 10 most beautiful villages in the UK (along with Lacock).
Don't be tempted to go looking for Cerne Abbas Giant (a hill drawing). This was a huge waste of our time.
With regards to the journey through the centre of Wales... it is nice, although we found it somewhat disappointing. The North of Wales was more interesting overall, although there are a few pockets of the very far south coast that are appealing. Not worth the big detour it would involve for you however.
Our highlights in Wales and this part of England were: Conwy, Abedaron, The Pheasant Inn, Lacock, Castle Coombe, and the Pontcysyllte Aquadact.
All of the places mentioned and many more can be seen in galleries on www.almostswiss.ch
Each photo show the location on a map under the photo, so you can see where these are in relation to your journey.
If you would like a list of all accommodation we stayed in (we could recommend them all) then let me know.
Have a great trip, and happy planning. Lynmouth. What a beautiful place (not mentioned in this tip)