Bryn yr Hen Bobl Neolithic burial chamber near Plas Newydd
Strictly speaking this is on private land and has no public access, but I generally think that if you’re quiet and discreet and aren’t disturbing anyone, then why not. The blocking stone in the entrance is very interesting because, although broken, it has two small apertures cut into it. It would also be interesting to know whether the inner chamber(s) go into the hillside or not.