Bath (day-trip) is a real time commitment — fit it in only if it's a priority.
Allow Full day.
A beautiful, walkable Georgian city — the Roman Baths, the Royal Crescent, Jane Austen everywhere. Worth more than a day really, but doable as a long one. Catch: it's ~1.5 hrs by train each way (or a long coach combo with Stonehenge), so the day feels rushed.