I would agree with Pam. The K and A canal would be my first choice, you get views of Bath you have never seen before.
Or go the other way on the Canal, down to Bristol.
Easy walking, no map needed, or walking experience, what's not to like.
Lots of places you can stop if you need to and hop on a bus back into Bath.
I've walked the Canal all the way from Reading (and London via the Thames) over many days as day trips by transit from London. With some planning you could thus do any stretch of the canal. For instance there is the Caen Hill flight of 29 locks straight up a hillside at Devizes- a true boaters challenge and a spectacular sight; or the Crofton Beam Engine Museum near Marlborough, Wiltshire- stationary steam engines which pumped water into the canal - they are in steam several days a year.