Cardiff Bay is very easy to go. It is very closed to Cardiff Central Station. You can take the bus or walk 30 mins from central station. It is free to visit, of course, and the scenary there is fantastic. You can spend a few hours there, no doubt. Take lots of pictures there. There are a lot of shops and restaurant and coffee shop there. So once you are tired, you can grab a coffee there, by sitting next to the seashore, enjoying the coffee and the sea at the same time. Different kinds of pubs and restaurant there, from high class to family restaurant. There are also a few historical buildings there, and there is a modern exhibition center there. The design of its outlook is so high tech, you definitely will fall in love with it.