Make a wind chime out of old keys and acrylic paint. - This site has lots of cheap outdoor/backyard ideas. I can't use most because I live in an apartment - but I love the old key to windchime and spray painted folding chairs.
I could make something like this for our patio with all the pretty beach pebbles I've been collecting! -Original pebble/rock art depicting a string of flowers (all natural materials including reclaimed wood, pebbles, twigs)