I second elderford`s nomination of Mitchell`s Fold stone circle:
http://www.themodernantiquarian.com/browse.php?site_id=139
as you will probably be passing within a few miles of it on your way to/from Newtown.
The Hoarstones stone circle is just up the road from Mitchell`s Fold, too:
http://www.themodernantiquarian.com/browse.php?site_id=2309
baz