I think it very much depends on if they will rent a car, how much time they want to spend travelling, and what they want to see. West wales is gorgeous, but it's a couple more hours of travelling from London and unless you were going to spend a whole day in Tenby as an example, you'd need a car to see the best of it...it's beauty is in all the little bays, beaches, villages etc
The following can all be done sensibly in a day without getting up at the crack of dawn, and staying out until late into the evening.
If they stayed close to Cardiff, I'd do a day in St fagans, https://museum.wales/stfagans/. You can catch a bus from central Cardiff.
A trip to Castell Coch, https://cadw.gov.wales/visit/places-to-visit/castell-coch and a wander around the woods up there, and an afternoon in Cardiff castle and a look around the arcades. Cardiff bay and the Barrage is a nice place to spend a sunny evening.
And a day trip on the train to Barry island, with fish and chips for tea!
It's not the finest of wales by a long shot, but a nice, easy, public transport friendly taste.