There are many coach tours for a day tour of Stonehenge and the city of Bath, which leave from London and should be booked in advance. We chose Viator, which is a Trip Advisor Company and includes entrance fees.
The tour happens rain or shine. Check the weather forecast and dress appropriately. It is about a 1-hour ride to Stonehenge, 1 hour from Stonehenge to Bath, and 1.5 hours from Bath back to London. The coaches have bathrooms. Food and drinks are allowed, but the tour company asks that guests not bring aboard strong smelling foods such as cheese and meat sandwiches.
Coach tours are the least expensive and time efficient way to visit Stonehenge and Bath. Make sure you read the fine print to confirm tour includes entrance fees. The company we chose, Viator includes entrance fees and gives other options as well, like including a stop at Windsor Castle.