I became aware of the musical version of "The Secret Garden" in my choral years. The group I was in sang "Come to My Garden" and I don't think that's the only reason that the song is my favorite from the show. Later, when I watched "It Factor" they featured Daisy Egan who won a Tony for playing Mary in "The Secret Garden" but was figuring out her place in Broadway as an adult. So the combination of a local production of "The Secret Garden" that included Daisy Egan (now playing Martha, the housemaid who encourages Mary to go outside and such) was a no brainer for me.
As is my habit of late, other than my great familiarity with the one song, I went in mostly cold.
One of the reasons for coming in with some additional knowledge is so that you don't spend the whole time marveling at the changes. The musical, for reasons I do understand, focuses much more on the adults. Also, books can get away with more internal obstacles, but theater often requires a more specific antagonist so Colin's doctor is now his uncle, who was also apparently secretly in love with Lily (Colin's Mom) but also maybe wishes Colin and Archibald would all go away and either leave him the house and property, or leave him to start over. (Yeah, I found Neville's motivation a little conflicting.) The ghost of Lily, along with Mary's parents, Mary's Ayah and some other folks who died of the cholera outbreak act as sort of a Greek chorus throughout the show, both narrating and also reinforcing the idea that these main characters of Mary, Archibald, and Colin are all living haunted lives.
Theater productions of books, even comparatively short books, always seem shorter, and the additional emphasis on the adults tilted that more. So yes, Mary still does discover that being outside and in the garden is good for her, and later for Colin, but Colin barely makes an appearance in the first act, and gets a comparatively short time in the second act. (The Shakespeare Theater production also cut "Round Shouldered Man".)
Mary's Ayah does not get a name, which is interesting because the male Indian spirit Fakir does.
So now that I'm done discussing changes, the overall idea of learning to focus outward to get through your grief is still there. Mary is still prominent, and the staging did some interesting things to demonstrate both the idea of the creepy dark house on the moor, and the garden that begins to flourish.
I left wanting to re-read The Secret Garden, and overall was really glad to have had the chance to see it.