Bath is small, pale, and almost entirely legible in a day and a half. The Romans built a temple and baths over hot springs. The eighteenth century built a resort in Bath stone and never really left. You are here for streets as much as for tickets.
It pairs cleanly with London on the GWR line, or with Bristol if you want a louder, younger city twenty minutes away and usually forgotten by the same itineraries.
See
The Roman Baths
The one paid interior that earns it. Go at opening. The water you look at is not the water you drink.
The Royal Crescent and the Circus
Free, and the reason the city looks like itself. Walk them early before the tour groups clap eyes on the lawn.
A climb to Alexandra Park or Bathwick
The honey stone only makes sense from slightly above.
Holburne or a bookshop hour
Small museums beat trying to add Stonehenge into the same afternoon.