Water filters!  Some can be recycled (Brita, PUR...), but what about Sawyers or even most that go into the fridge?  In addition, it's disappointing that Sawyer for example doesn't sell the filter unit alone.  You get the whole system and all those parts that you may not need and can't all be recycled.  I keep coming up with things I can use the syringes for, but I really don't need them and don't need a straw, additional bag each time I buy one.