I had a fantastic day out in Cardiff, I would highly suggest getting a ticket for an open top bus to see the city as it is way bigger than I thought it was going to be and it stops at all the top attractions like Cardiff Castle, National Museum Cardiff, Falkland Islands War Memorial, Cardiff University and so much more. It's not very expensive either and seeing as it was raining on and off on the day we were there, we may not have been able to get off and see all of the places up close but it was good to just see the whole city.
Pierhead is a fantastic bit of history that I had great pleasure in walking around. Construction was finished in 1897 when Cardiff Bay became a huge port for ships and the building was used as offices. It has a clock tower which some call the Welsh Big Ben. Have a good look round at the exhibits and displays they have. We watched a brilliant presentation on the history of the building in the big main hall that was very informative.
Cardiff Bay is great if you want to go on a little boat trip around the bay, people watch in one of the many restaurants or go shopping in the shopping centre. We went for breakfast in The Mount Stuart which is a Wetherspoons pub. It has a huge glass walls so you can enoy the view of Cardiff Bay while you eat and drink.
Roald Dahl Plass isn't just great for Roald Dahl fans but it is great for Doctor Who and Torch Wood fans (like myself) as it is a great landmark to take in the atmostphere and chill out for a little while.
Wales Millennium Centre has lots of great art exhibits that we had fun walking around. They have loads of shows on if you are around in the evening.