I went for the day in 2019. There's an interesting indoor market and some good architecture. It was raining when I arrived so I went into an Italian restaurant for some hot carbs. I just walked around for the afternoon taking photos, had done no research into the city beforehand or since, Wales was on my bucket list to visit.
I have in-laws in South Wales, but I've not been into Cardiff for years... it's probably the best place in the area for live music though, from the big Arena to medium and small venues. I can't imagine you'll have too much trouble finding something, although things are still a little weird post-youknowwhat.