I've done that tour and enjoyed it immensely.
Canterbury: We went down 2 nights before so we had a full day and another half a day. I found plenty to do in Canterbury even though I think you have a part of an afternoon on your own there. We walked over to St Augustine's Abbey (https://www.english-heritage.org.uk/visit/places/st-augustines-abbey/), walked along the Stour River to the water meadows, poked around a bit.
There was one place we didn't get to as the opening hours didn't match our time, and that was the Battle of Britain Museum in Hawkinge. There is a local bus route that goes near there and seemed doable if the times had lined up.
http://www.kbobm.org/
Bath: One of my favorite cities! I know many on the forum feel there is not much to do here and it can be done as a day trip but I disagree. I'd say a resounding yes to spending 2 days (is that 3 additional nights?) there. The tour has been re-formatted since I took it which is good. We rolled in to Bath at about 530PM so had no tour time in Bath. Now they've got it so you arrive early afternoon. There are the Roman Baths, Bath Abbey, Holbourne Museum, Royal Crescent, Circus (Regency era townhouses facing onto an oval), #1 Royal Crescent Museum, Walk along the river and along the Kennet and Avon Canal. I enjoyed Prior Park which has a Georgian landscape feature I was interested in but it's a bit of a walk (uphill!) from city center.
If you want to do day trips from Bath, you could consider using MadMax which is a small local company with tours to the Cotswolds, etc. I looked at their webpage recently and of course they don't have nearly as many tours as they used to but maybe they'll be adding some in as the summer progresses.
You will have spent time in Salisbury and at Stonehenge on the tour so no need to double back there.
You'll have a wonderful time!
