Wild near Cardiff
I have stayed overnight whilst traveling to and from Anglesey and the south at a layby next to the Llewin Onn reservir, noth of Myrtha and about 13 miles south of Brecon; a loop of the road and behind a few trees, so passing traffic is not a problem. Nice lawn, picnick tables and a path down to the water. There are other laybys on both sides of the A470. If you want to 'do' Cardiff it is a bit distant at about 40 miles but a good transit stop.
Hope this helps.
Jones'y