If you ask me for a city to visit on a day trip from London, Bath will definitely be my suggestion.
Bath is the perfect town to spend a day and disconnect from London.
You can rarely see any modern buildings or concrete towers under construction, all you can see are Georgian houses built of the characteristic yellow stone.
The downside side is that there is not much to do for more than a single day excursion.
You can walk through the streets and look around the beautiful houses, most of them with colourful flowers in their entrances. Circus Square and Royal Crescent street are the most famous pinterest places to take a photo and imagine you are in the Georgian Britain. On a sunny Sunday you can even listen to music at the Royal Victoria Park’s bandstand.
The Roman baths are nice. The main pool is romantic picturesque, but being honest, when you come from a Roman City like me, it is difficult to be impressed.
The Modern Spa is a good place to relax and recover your energy. Just be aware that it can be full of hen parties and it can be difficult to have a space in front of the mirror of the changing room. I strongly suggest to bring your own hair (with a sigh of frustration).