Stonehenge, Wilts.

English Heritage.
One of the iconic English monuments. It is easily found at the A303/A344 junction and you could make it a stop en route to the south-west. There have been plans to expensively re-work the surroundings and visitor access, but when I visited in 2007, one had to park across the road from the stones in the EH car-park, and then cross under the road via a tunnel. One could walk around the stones in a circle but not walk amongst them. The site is open daily. (Admission charge) A free audio tour is available.
Druid myths apart, the site is impressive, the standing stones are colossal and the prehistoric workmanship amazing. A number of fallen and unstable stones were re-erected in the 20th century to restore the circle to more like its ancient complete form.
The ancient monument enthusiast will find other monuments in the same area.