St. Fagans is the National History Museum of Wales. It includes a large open-air musuem as well a castle. The open-air museum frequently has live actors and activities such as blacksmithing, throwing pots, or whatever else you can find in a Welsh village. There is livestock, herb gardens, even a mill, and a bakery that bakes bread made from flour in the mill. You can easily spend a whole day here. This site is not only very well-done, fun, and educational, it is also free!
There is something here for everyone. The grown-ups liked the Castle gardens, complete with ponds, rocks, and acres of flowers, green grass and trees. Baby Isaac liked the farm animals, and got to pet some friendly sheep. Chloe and Michael chased around in the cock-fighting pit. The scones with clotted cream and jam were delicious. The Welsh houses were fun to explore, like seeing how people and animals used to live in one house together and part of their heating technology is body heat from the animals. Houses are very dark and low, so watch out if you have transitional glasses. The castle is more of a manor house than a castle, and there isn't much inside other than a good pit stop for a bathroom break while exploring the charming grounds outside.
All in all, a very impressive place.