Well, technically, the indigenous Hawaiians didn't wear grass skirts like they have today nor did they wear the Hawaiian wear that is so popular because that is due to missionary influence. Yes, ti leaves are usually only for dancing hula. (There's another DIS member here who speaks fluent Hawaiian... where are you?

)
As a part Hawaiian person myself, I am all for authenticity, but to a certain extent when we don't have ti leaves here on the "mainland" or the usual get ups.

I certainly won't dance the hula or walk around topless which is the traditional way. lol
I just had Thinking Day with our troop. We wore grass/raffia skirts, pareaus/sarongs (Hawaiian print or floral fabric wrapped at our waist or like a coverup) though that is not indigenous, leis (actually, it is
na lei) made of fake flowers and some wore shell leis, and muumuus or Hawaiian print shirts (not indigenous either - but what can you do? lol).
Use what you have.

I like those ideas of using crepe paper or paper bags because it is using resources wisely and that is the Girl Scout way.
PS I'm also a leader/Recruiter/Age Level Consultant/Leader Support Team member of Girl Scouts... Just a PSA... did you know that Queen Lili`uokalani together with a teacher of my alma mater, Kamehameha Schools, sponsored the first Girl Scout troop in Hawaii in 1917? That's incredible because GS was only 5 years old back then!