Bath area in September

That does seem strange, doesn't it? Rome2Rio.com suggests that you may need to take a train to Bristol-Temple Meads and a bus from there to Wells. You can follow the links given by Rome2Rio to be sure it's correct about the frequency of the trains and buses. I have known it to be wildly wrong.

If you visit this web page

https://www.firstgroup.com/bristol-bath-and-west/routes-and-maps/route-maps

and download the timetables for 173 and 174 you can see buses between Bath and Wells (they run daily).

Just did the following walks a couple of months ago:

Bath Skyline / Walk to the View (3 miles)/ Sham Castle Circular (5 miles): some hills, but great views of the city; swing by Prior Park landscape gardens too; the entire Skyline walk is about 9 miles total but you can do parts of it to split things up and/or avoid the steeper hills; there are different guides and maps explaining options. https://www.nationaltrust.org.uk/bath-skyline http://www.bathnes.gov.uk/sites/default/files/sitedocuments/Streets-and-Highway-Maintenance/FootpathsandPublicrightsofway/sham_castle_circular.pdf

Bradford-on-Avon: you can arrive by bus (or much faster with the train) to the town center (also well worth a look around!) and then do a really nice, easy walk along the canal path or the river to Avoncliff. There is a pub at Avoncliff with tons of outdoor seating by the river with a view of the aqueduct that allows the canal boats to cross above the river! After a pit stop, you can either walk back (roundtrip is about 3 miles), continue on various extensions to (google Avoncliff walk for other ideas/variations) or hop on the train in either direction.

Bathampton to Bath - a bit shorter, but nice pubs and scenery along the way; you can take the bus to Bathampton or walk roundtrip to make it a circular route; this can also join up with part of the Skyline walk around the university area

This might also give you some ideas (have a look at the one "Why Not Walk? Lansdown" which looks bus-accesible http://www.bathnes.gov.uk/services/streets-and-highway-maintenance/public-rights-way/exploring-countryside/easy-going-walks

Thanks so much for the link to the 173 and 174 routes. That's the one I was looking for because I remember how scenic the decent into Wells is.
Thanks too for the walking tips. The canal walk from Bradford sounds good.
Cheers!