Salisbury is about a 90 minute train ride from Waterloo Station. Just outside the train station you can catch a bus to Stonehenge. You'll have a bit over an hour before the next bus comes, which is plenty of time to walk around the stones. Catch a bus back to Salisbury (or if you're adventurous, ask the driver to let you know the stop for Old Sarum. It's a bit of a walk, but you can see the original site of Salisbury, with a nice view over the plain, too.) Back in Salisbury, be sure to visit the Cathedral to view the Magna Carta and the exquisite interior. There is a nice attached cafe and gift shop. Back in town, The New Inn is decidedly NOT new--16th century, if I remember correctly. If it's nice out, their garden is a perfect spot for a pub lunch or a pint. If not, now that there's no smoking in the pubs, it's a cozy, pinch-me-I'm-in-England sort of pub.