National Botanical Garden of Wales

The National Botanical Garden of Wales has (they claim) the largest single span greenhouse in the world. The site is set in a large park where you can go for walks of up to 1 or 2 hours. The less adventurous can walk or (if you have an excuse) take the buggy ride from the entrance up to the Great Glasshouse, and make your way back via the Double Walled Garden, Tropical House and whatever catches your eye on the way. The site owners say that to see everything requires an all-day visit.
The Great Glasshouse contains endangered plants from a number of regions with a Mediterranean climate, including South Africa, Australia, California and Chile as well as the Mediterranean basin. There are a number of specialist gardens and exhibitions scattered around the site.
Worth a visit if you are interested in plants, particularly exotic ones. When I visited at the end of June, there were not many flowers to be seen. Some familiar vegetables were growing in part of the Double Walled Garden.
Some re-purposed older buildings remain on the site, but not the mansion, which burnt down in 1931.