Stonehenge is about 1 hour from Bath and most people spend about 1 hour at the site. It is very close to the Medieval city of Salisbury with the magnificent Salisbury cathedral. The cathedral has the highest spire in the UK and houses the best of the 4 remaining copies of The Magna Carta. Combining these 2 sites is very easy and would require about 4 to 5 hours.
Alternatively, you could combine Avebury and Stonehenge, if your passion is ancient stone circles. Avebury being 500 years older than Stonehenge. Again about 5 hours required.
You could add any of the places above to the half day suggestions, but that would take you nearer to a full day trip.