My hometown! It's great, loads to do. What sort of thing do you like? Stay centrally if you can, then you can walk everywhere you'd need to go. Cardiff bay is only a train ride away (10 mins or so from Cardiff Central or Queen Street?) So easy to go to if you fancy seeing the sea, Welsh Assembly building, etc so better to be central most of the time I would say.
Dh and I really enjoyed the millennium stadium tour when we were last there, and the Castle is great especially for the views of the city from the top of the keep. There's also the national museum of Wales centrally.
Shopping wise everything is in the centre, shopping centres and a few main streets (Queen Street and St Mary's street primarily). Massive John Lewis if you like that sort of thing! I'd also suggest checking out the arcades which run around the st Mary's street area, they are really special and there are some great shops there. Also walk through the indoor market and buy some Welsh cakes! My favourite shop to head for when I'm back is Kitchens on St Mary's Street, it's a treasure trove of baking gear! Also Wally's deli in Royal arcade is wonderful and the cafe upstairs is great for lunch or a pitstop. The New York diner in Castle arcade does wonderful enormous American style sandwiches too which I definitely recommend - they've been there since I was a child, don't think I've ever finished one!
Restaurants and bars have changed a bit since I last lived there full time before university though. Dh and I had a night out last year though, we went for tapas over near the stadium (can't remember name!) And really like the Cambrian Tap pub which sells Brains craft beers - really good and hard to find elsewhere. They'll sell you bottles to take home too. www.sabrain.com/pubs-and-hotels/south-wales/cardiff/cambrian-tap/ it's at the corner of Caroline Street and St Mary's street. Hopefully a current Cardiff resident will come along soon with more suggestions. They definitely have all the chain places, though always a shame to opt for that all the time when you're on holiday!
Hope you have fun! This makes me think I should go again soon - maybe when DS is a little bigger!