I was browsing Pinterest with my three year old son on my lap and an I-Spy Bag was on the screen. He was so excited and asked what it was, so I decided to make one for him. I used fabric from two shirts that my eldest son has grown out of, and I backed it with an end of medium weight denim. The clear vinyl is from some packaging, and I reinforced it with topstitching.
I filled the bag with four cups of rice, which filled it about half way. The difficult task was finding trinkets to fill the bag, but I was able to find twenty pieces that work really well. The finished bag measures 9 x 10.5 inches with a 4 x 5 inch vinyl window. The large window will make it easy for my son to find the trinkets, while the use of rice makes a bit challenging to see all the pieces.
If you are thinking of making one with light or medium weight fabric, I suggest using fusible interfacing or double up on your fabric (which I did). Using a medium weight fabric at the back of the bag is great for wear and tear. I think that the new toy is going to make car travel a little more fun for my little guy.