I will be travelling from Stow on the Wold in the Cotswolds and have 4 days to explore Wales in Mid May. I will have a car. Do you recommend North or South Wales? Any recommended accommodations? What are the top 5 sites or activities in Wales? Anything I should stay away from?
In 2015 we did a similar trip as you are plannng, also with a car, coming from Shipston on Stour, staying at a B&B in Betws-y-Coed in northern Wales for 5 nights. This has been our only trip into Wales except for a day trip to Tintern Abbey from our Cotswolds base, so I'm not familiar with sites in the south.
During our 4 days in the north we visited Harlech, Conwy, Carnarfon and Beaumaris Castles, toured the Dinorwig Power Station located in an old slate mine, visited the National Slate Museum in Llanberis, spent most of a day taking the rack railway up Snowdon Mountain and hanging out with rocks and sheep at the top, visited Port Meirion (a private Italianate town) just outside the west edge of Snowdonia National Park, and had a speedboat ride on Menai Strait in Anglesey.
We absolutely loved the slate in northern Wales, so much so that if we build another house we're going to finish it in that same style and colour of slate.
Good luck with your planning,
This is the website of the government run tourist information and should give you lots of info:>http://www.visitwales.com
Across a narrow band of south Wales is the former coal mining area - which is best avoided in my opinion - although Caerphilly Castle and the Big Pit coalmine (free entry) are well worth a visit. Including Caerphilly - the Cardiff area has 4 castles and is certainly worth at least 1 day but preferably 2 days.
You could of course, head NW from Stow-on-the-Wold to Snowdonia (NW Wales) & visit the places previously mentioned but I would also favour leaving the Cotswolds for Tewkesbury (see the Abbey) and then take the M50 to enter Wales near Monmouth - the Wye Valley in this area is very scenic - especially near Tintern Abbey. From here, you could either head SW to Cardiff or NW via the A40 to the Brecon Beacons area for a couple of nights. (The A40 can in fact be used all the way from The Cotswolds as an alternative to the M50 route). http://www.breconbeacons.org
Google the places mentioned for more info. 4 days is not really long enough to do much of Wales. If you want to spend more time sightseeing instead of driving, confine your trip to the SE of Wales = lower Wye Valley (Tintern etc), Cardiff area & the Brecon Beacons. If you want to see more rugged mountain scenery - head straight for Snowdonia in the NW. Ideally, you would need 10 to 13 days to see the main highlights of Wales. It would take you the best part of a day to get from Stow-on-the Wold to Snowdonia (NW Wales).
We enjoyed the places the other respondents stated, so it's really a matter of what you enjoy doing. While we spent more time in North Wales, the Cardiff area is closer to where you're starting, so you could see more places by spending less time traveling. When in Stow on Wold, ensure you walk around to the back of St. Edwards parish. There is a nice church door framed by a couple trees. Makes for a nice photo.
In terms of events, we're attending the Royal Welsh Spring Agricultural Show on May 20 (also occurring May 21) at Llanelwedd, which will be a kind of large agricultural fair in mid-Wales. About 20,000 people attend, the summer show is larger but this sounded appealing. We like fairs at home, so thought it would be fun to attend the same type of event in Wales and talk with people informally. I wouldn't say this is a top site or activity; we like attend/do things that are distinctive events for the area we're visiting.
We're in Wales starting in mid-May as well. We're choosing to spend more time in the north. We're planning to see: Bodnant Gardens, Conwy castle and Plas Mawr (fortunately or unfortunately will be there during Conwy Pirate Weekend), Great Orme Mine, Anglesey with Bryn Ceili Du and coastal walks and possibly Beaumaris castle, and Caernarfon. We aren't tapping into Snowdonia due to lack of time. There are a ton of things to do and see in the north.