Bryn Celli Ddu on Anglesey is well worth a visit, despite being fenced in. Check out the tree-trunk like pillar in the chamber - fascinating.

http://www.themodernantiquarian.com/site/352/bryn_celli_ddu.html

And I second Postman's suggestion of Penmaenmawr, but make sure the weather is OK, it can get pretty wild and windy up there even on a relatively fine day.