I did a Greyline tour to Stonehenge and Bath. It was great. Enough time spent in each locale and didn't feel rushed. It was really cool to see Stonehenge and Bath. Windsor would be fun too, but as others mentioned, that's a tour in itself. Frankly with out a private car and tour guide, I don't think you could do all 3 in a day.
The following is based on fun and being fam friendly for the pocket book.
One thing I loved in London are the walking tours. They are low cost and amazing!!! www.walks.com
I couldn't believe how big London is. Wow! everywhere there is cool stuff. Harrods. The Harrod's food court is amazing. I had tea at the Ritz for my birthday (dress code but amazing experience that you cannot get anywhere else and very London)
One way we got the lay of the land was the hop on hop off bus tour. Great way to see the town. For inexpensive lunches Prete e mange. Love them.
The cabbies are so amazing. They know where to go if you ask and know what you are looking for. We found a fantastic fish and chips place-- inexpensive and knew it was popular when people would line up for take out.
There is a Princess Di walk in Hyde Park. Hyde park hiking on a Sunday is amazing--everything is closed for the most part until the afternoon, a great way to spend a Sunday morning. Speakers corner is not to be missed. The Peter Pan bronze statue is really cool.
You may be able to score some theatre tickets at the half price place too if you are so inclined.
The tube makes traveling quick and easy around town, too. And the double decker buses.