I have never made an I spy bag before. They were really popular for a while and then I haven't seen them much since. I spy bags are great for cleaning out old buttons and random items: pennies, army guys, pom poms, little animals, Barbie accessories, etc.
To make an I Spy bag, you'll need to acquire 12-25 items that will fit in a bag. You'll need a bit of flexible, but durable vinyl. You can get it for a small price at the craft store or product packaging (think those bags that sheets and blankets come in). You'll need some polyfill beads or rice. You'll also need a small about of fabric.
I made this I spy bag about 7" by 7." You can make them square, like mine, or you can do some like those found here.
I found a fun idea here about putting a pocket on the side for housing the I spy find list. I forgot to put mine on like she did and had to stitch one on by hand after.
I also like the idea of taking a picture of the items in the bag for kids who can't read.
This one was for a gift and I only had time to make one before I had to go, but I have the stash to make 2 more, so I'll have to get those done too. The trick will be getting my son to share it with me so I can do some finding too.