You could buy recycleable plastic cups and ask the people to stay and drink there drinks then you can wash them out and reuse them.  Or you can advertise for people to bring there own reuseable water bottle to fill up so we can save the planet.  You can advertise it as a green lemonade stand-something for the planet. I'll keep thinking.

You could also just have the customers buy the cups that you are giving out. Maybe give the donations from the cups to a good local organization. Get creative, it will be fun for you and your daughter!

I think that this could be a really fun thing to do with  your daughter. You could color the cups green or have green wrist bands for everyone who buys some from you.  You can also set up recycling bins to encourage recycling of the cups and have a sign having some facts about recycling.  The possibilities are endless.

just putting thought into make her lemonade stand green is so awesome.  she is one lucky girl to have a teacher like you :)
the stand could raise awareness to reuse, recycle, organic, gmo.....
i wonder if there are cups that could be composted?   hmmmmmm

I know that there are plastics that come from recyled plastic. I have seen them in Whole Foods.  I would think that they would also carry cups in their product line.