I think you fall in love accidentally, and you even don't know why, as there are a lot of lovely cities in Great Britain, but personally, I found myself completely happy and satisfied with Caerdydd (let's call it the Welsh way).
Maybe because of the weather, helping a lot, maybe because of the atmosphere. Don't know, but Cardiff has the perfect dimension of a big city and a kind of everything to feed your expectations.
So you will enjoy the Castle and medieval side (there was also a 2WW exhibition when I went, that was absolutely interesting, just a bit dark to give the impression of fear and instability of that time), the park next to it, so beautiful at sunset.
From the Castle tower, you can spot the surrounding hills and far away ancient castles that would be worth a visit.
You will spot also the Millennium Stadium, famous rugby and sport venue.
The city centre is a mix of traditional welsh streets, shopping galleries and pubs and a modern area with shopping centres and cinema.
If you have time, enter the Cardiff Museum, for a time journey on the city history from the first Roman fort.
Moving towards Cardiff Bay area, you will feel like a sailor ready to set the ship to new discoveries, enjoy the Millennium Centre and the bright and white buildings close to the sea, restaurants and bars where you can taste a real welsh cake with a cup of tea.
There is also a curious and nice Norwegian Church, which is an art centre actually.
I really recommend then to relax at the Alexandra Garden and admire the colors of the flowerbeds and the plants. In spring they are glorious.
Not far, there is the National Museum. If you love art, please go and stay breathless in front of The Kiss, by Auguste Rodin.
Croeso y Cymru!