London Walks runs a one day trip by express rail and bus to Salisbury and Stonehenge. Very reasonable and obviously takes you to spectacular sights.
Sounds great!
http://www.google.com/search?q=monuments+of+wiltshire&sa=X&oi=image_result_group&tbm=isch
http://www.touristnetuk.com/southern-england/wiltshire/attractions/heritage/castles.asp