I built my kids "the box", which had a similar concept. It was an upholstered box with a frosted acrylic top and a camera mounted inside facing up. You would put cards on top of the box and it would recognize the symbols and take an action, like play a song or read a word. I used large printouts of Data Matrix, as I found that faster and most accurate to decode. The front (up-facing) side would have a colorful design.

In the most advanced version I had it read letters to read a word aloud (eg, a spelling tool). All so I could reenact the scene from Sneakers where they rearrange Scrabble tiles from SETEC ASTRONOMY to TOO MANY SECRETS :)