Stonehenge remains a mystery. Perhaps it was built for religious purposes or by aliens. Whatever you believe, Stonehenge is worth a visit, though very touristy. There is also not much to do at Stonehenge besides, well, see stones, as it is really in the middle of nowhere. You can take in the beautiful fields surrounding the site but it is best to plan other stops on your visit to make the trip more worthwhile.
Many tour companies offer trips to Stonehenge, Bath, and/or Windsor Castle. Bath is the perfect add-on to a Stonehenge trip. It is best to make your own way to Windsor Castle so you have more time to explore freely.