Let me warn you. Once you make an I Spy quilt, you will have unique fabric that begs using so you will buy more fabric to go with it and make another. Then you will trade fabric and make another one. Then you will see one that you really like and make one like it. Then you will need more unique fabric.
The one I've like most to date is the one I haven't made! I like the idea of having two of each block with the matches set on point around the outside but haven't done it. My friend makes I Spy quilts that look like a colour palette. I'd like to try that but haven't. I keep hoping each I Spy is my last.
Decide what size blocks you want before you begin collecting fabric to help ensure you don't end up with pieces too small to use.