Some great suggestions there. The Gower has some great chambered tombs, mostly easy enough to get to:

http://www.themodernantiquarian.com/site/111/maen_ceti.html
http://www.themodernantiquarian.com/site/14497/nicholaston_long_cairn.html
http://www.themodernantiquarian.com/site/6295/penmaen.html
http://www.themodernantiquarian.com/site/107/parc_le_breos.html
http://www.themodernantiquarian.com/site/3543/sweyne_howes_north.html

If you're prepared for a longer trip then there are some brilliant, easily accessible chambered tombs in Pembrokeshire, for example:
http://www.themodernantiquarian.com/site/113/carreg_samson.html
http://www.themodernantiquarian.com/site/112/carreg_coetan_arthur.html
http://www.themodernantiquarian.com/site/116/llechydrybedd.html

And a fine stone row:
http://www.themodernantiquarian.com/site/118/parcymeirw.html