I've flown from Cardiff airport quite a few times, but never needed to stay overnight there.
The airport is pretty small with minimal facilites, but I understand there have been some improvements lately, so there may be more there now. Last time we were there, there was just the one row of check-in desks, then up the stairs to the departure lounge with a self-service restaurant, bar and a newsagents. Once through to airside there's another bar, duty free shop and a couple of other shops selling the usual airport bits and bobs.
Coming back on arrival is usually very quick and straightforward, and you'll be back out into the Cardiff damp in no time!
I have flown from Cardiff loads of times. It is not a big airport, there are about 30 check in desks on the ground floor, on the first floor there is a small games area, a small cafe, a bar and a papershop, oh and some toilets.
You enter into departures from this level as well, once inside, there is a fair size duty free shop, a pub, a cafe, and a paper/sweet/book shop, so not very big at all. Of course there is plenty of seating and toilets and phones.
The gate numbers are about 1 - 12 I think, so you dont have very far to walk to your flight.
There is a brand new Holiday Inn Express right next to the airport, the link is below, hope this helps: