On the South Shore, there's some really nice, low key hiking at World's End in Hingham. It's a park that was originally laid out as a housing development by Frederick Law Olmstead, but the houses were never built. It has beautiful views of Boston, and beaches, meadow, and forest. Also in Hingham is Wompatuck State Park and Weir River Farm. They have story hour on Wednesday mornings and are also open Saturday morning.
In Norwell, the South Shore Natural Science Center has lots of kid friendly trails, as well as a small indoor nature center. They do various events which are always fun. Also in Norwell is Storybook Cove, in the Merchant's Row mall. It's a small independent children's bookstore. They have story and craft times. Another good local bookstore is Buttonwood Books on 3A in Cohasset.