I had a particular conversation over and over again while doing Mighty Ugly at the Vancouver Mini Maker Faire last year*, and it surprised me every time. It went something like this:
Parent: Look at all these pop tabs/bottlecaps/twist ties/doodads! How clever to use these for crafts instead of throwing them out!
Me: *blinks*
Me: *realizes response is necessary* I just keep a bin next to the recycling and put appropriate stuff in there instead.
I’ve always used scraps and otherwise ready-for-recycling materials for Might Ugly, since it’s important to the project that the materials not be in any way precious. Of course, I also frequently raid my saved-from-the-trash haul to do things like make bottlecap pincushions, and with Owen now old enough to get into crafts, I’ve been raiding it for toddler projects, too.
But in case you need to get matchy-matchy and crafty-crafty to collect this kind of stuff with  your kids, here’s a post with info on making a recycling station for just this purpose: DIY Recycling Center For Kids | Handmade Charlotte.
I’m involved in a volunteer project that requires tons of craft supplies and other things, and everyone was so surprised when I said I would go to the dump to see what I could find… I think almost every dump (transfer station) has a shed where people leave stuff they don’t want but is still in useable/fixable condition. Today, I picked up some glass vases for a decoupage project. :)