Windsor Castle & Bath

You could do it on your own. Take the train from London Paddington Station to Windsor Eton & Central it’s a 35 minute train ride, you change trains in Slough which is simple. When you arrive in Windsor exit the train turn to your left, walk up to High Street and there across the street is the castle.

From London Paddington Station to Bath is a 1 hr 22 minute train ride. Bath Abbey is a 7-10 minute walk from the train station.

You can book tickets on nationrail.co.uk

Bath really needs close to a full day, if you haven't been before. Windsor can we 'done' in a 1/2 day. We'd often go on the day we had theatre tickets- leave on an early train (no need to book this one ahead as tickets cost the same on the day of as in advance) and return soon after lunch, then hit the markets and eat supper prior to the theatre. There is good shopping in Windsor, too. For Bath, we'd also take an early train (this one you want to buy ahead as it is very expensive on the day of, but discounts are very good about 12 weeks prior to your journey), arrive just in time for an early lunch, visit the places we were interested in, shop, and in summer do a cruise on the river. Then we'd take a train back to London, arriving in time for a late supper.

If you really must combine them, get to Windsor very early, and if you want to tour the castle, see if you can book a timed ticket for when it opens. We loved watching the changing of the guard here rather than at Buckingham Palace- much better sight lines and smaller crowds. Then head to Bath prior to lunch (maybe purchase something to take and eat on the train). Do what you want to do in Bath, and plan on taking a very late train back to London (probably eating supper in Bath).

I just visited Windsor, Windsor Castle and Eton. Been to Bath on a previous trip.
I think 3 hrs is plenty of time for Windsor Castle. I did not care for Windsor, the town. Eton was lovely but if i really wanted to do the castle and Bath, i’d skip the walk to Eton. A full day in Bath is best, but you could do WC and Bath in a day if that’s all you have.

Trains to both places leave from Paddington station, but simply due to the duration of the train ride to and from Bath, I personally wouldn’t try to do both on the same day. As stated, if you leave early enough, you can visit Windsor Castle in half a day, if need be. You may not get to walk around town very much, but you’ll get to see the castle. We bought our tickets to Bath directly from the gwr.com website for an upcoming trip, 5 weeks in advance to get a decent price. Weekend prices are cheaper than weekday prices, and non rush hour weekday prices cheaper than rush hour prices. For our trip to Windsor a year ago, we just bought them at the station since we didn’t know which day we were going there.

I found this one: https://m.viator.com/tours/London/Private-Day-Tour-to-Bath-and-Windsor-Castle/d737-137859P2

There’s also this one if you combine it with an airport pickup. https://m.viator.com/tours/London/Heathrow-Airport-Arrival-To-Bath-Via-Windsor-Castle/d737-67271P6

With only a few days, I’d pick only one and that would be Windsor Castle. It’s easy done as a day trip. Bath punches way above its weight among Americans. Out of all the U.K. friends I’ve met, I’ve only known one who has been to Bath and that was for a Romantic getaway. I assume most of the Bath popularity is due to Rick Steves.

Is it your first time to London? If yes and you only have a 'few'(3-4) days, i wouldnt do any day trips, London will have more than enough to keep you busy!

"I assume most of the Bath popularity is due to Rick Steves."

I started going to Bath WAY before I ever heard of Rick Steves. That was the 1980's. I went back in 2013, again before I knew Rick's opinion of it and have been back a number of times since. I'd say it's popularity is due to the Roman sites AND to the Jane Austen/Regency era connections/architecture. In fact, reading novels based in the Regency era is where I first became aware of Bath.