There's stuff in the Hafren Forest, although my walking feet failed me on the day: notably Carreg Wen, near Llanidloes, a very beautiful place, where the young and sprightly Severn slides over a rocky bed.

Another place which blows my mind, is Walton's pallisaded enclosure, although it's not really much of a 'sight' of a site, is (probably according to me!)one of the most important enclosures in Europe, it's near me Mum and Dad's house so I can be forgiven for raving:
http://www.jungsteinsite.de/2000_gibson/hindwell.htm

Powys/Shropshire is a beautiful part of the world to be sure, and kept this shutter-finger busy for years :-)