Our last visit was to the Arboretum de Balaine. It's a bit farther South from where we were staying, so not strictly on the way home, but we incorporated the visit into our journey back.
It's a very different garden from that at Apremont, with a feel much closer to a big (English-style) public park than a private garden. The route around the garden is signposted, and visitors are expected to follow a specific path that takes in all of the important features. The ways are fairly narrow so perhaps this constraint is for when the garden is busy, although we were the only visitors that morning, as far as we could tell.
An average tour around the grounds takes about two hours, and we found it very relaxing.
Definitely worth the detour.