I like to leave handcrafted/upcycled items. Once I left a red and white quilted blanket that I made at a cache near a picnic area. I sewed an old red plastic table cloth on one side so that it would be waterproof if someone wanted to lay it on the ground. Another time I made a bird house from an apple gourd and hung it from a tree (it was covered in plastic so birds wouldn't nest in it). It adds a little extra eco-friendliness to an already great nature adventure.