A great thing to do while visiting South Wales, whether it's winter or summer, rain or shine, is to visit the National Botanic Garden of Wales. January is free to enter and even though it's cold outside and not much is growing outside, the indoor part is beautiful and colorful.
This time of year the snow drops are in bloom and they can be seen everywhere outside. Inside you can observe flora form around the world like South Africa, Australia, USA, Chile. It's a lovely family outing. Also picnicking is allowed inside.