Slurpees come in plastic cups. End of story....But, I'll continue for sake of the story...they can be downcycled -if we're lucky - i.e. if the individual getting said slurpee puts their waste in the proper receptacle, and that's also assuming they get the large or the xtra large size as the small and medium come in those ever-present plastic lined paper cups that can neither be downcycled as plastic, nor recycled as paper! What a product.
So, to that I say.....
Slurpees in your stainless steal water bottle, or re-usable coffee mug - or any other sort of re-usable!
Just as I did with a jugo juice, I did with a slurpee and it worked out great. Now all I need is a glass straw and I'm set.