Treasure Island and Reno happen to be my two favorites.
Especially Tresasure Island, so much so that I have acquired a small collection of memorabilia from the Golden Gate Exposition that Charlie and Jimmy visited there (I have a guide book, some tokens, a locker key, etc).
To go a bit off topic for a moment, if you read mystery fiction, you may want to get a hold of Rupert Holmes' novel Swing (2005), which is set at Treasure Island in 1940. Another novel set there is the contemporary mystery Murder Loves Company. In my imagination Charlie Chan and the protaganists of those two books are investigating crimes there almost simultaneously, perhaps even bumping into each other.
This was the first I heard of "Secret" not being part of the Volume 2 set. Do you (or anyone on this board) know the reason why Fox chose to upset the sequence of films on the DVD sets? I read Rush's previous posting that Volume 2 will likely be delayed....