There are lots of bits and bobs in the area. Mostly wrecked. However...

The Black Mountains has a variation of Cotswold-Severn tombs (angled side passage entrance) and its an inland group. They are dotted about where you are visiting but Arthur's is the best preserved.

The stone circle is the fun and games of it being the largest in the region but the worst preserved (one stone standing). It's up on Hay Bluff where they filmed the beginning to American Werewolf in London.

Penywyrlod Long Cairn is worth a visit. Rediscovered in 1973. National Park not too sure what to do with it. Largest in Wales. We also have the highest elevation Standing stones (standing and fallen) and the largest (on private land but if you contact prior you will probably get permission).

In recent years they rediscovered an unmolested long cairn that is nearby to the stone circle. Not on maps.

FoArthur's I think the recent excavations took place in the field across the road from it.