For a proper family day out if your coming to S.Wales I’d do.
First stop Castle Coch, not a proper castle but great fun, you’ll have seen it in dozens of tv shows and films.
Caerphilly castle next only 15mins away, but before you get there park at the snack bar on Caerphilly mnt. Cross the road and walk up to the trig point, you get a great view of the castle it’ll help you get a sense of the true scale of it.
Once you’ve done that especially if you’ve got kids, drive another 20 mins up to Nelson and visit Llancaiach Fawr manor house.
It’s a 16th century manor house, the staff are all in period costume and do an interactive tour of the house. It might sound cheesy but it isn’t, kids especially will love it. That really should be enough for one day.