Found this great site icanteachmychild.com and this fun quick project. I had it in mind for the 3 year old but our 5 almost 6 year old thought it was a little more interesting. I made mine with pictures on both side to make it a little harder to solve. If I did make them again I will try to glue/modge podge the pictures to the blocks for durability. Over all a fun use of building blocks.
I think the block size is about 1.5" x 2" so the pictures need to be sized just smaller than that to fit. We just used Microsoft clip art and typed in the animal categories we wanted for this set. This would be a fun baby shower gift...we maybe a toddler who no longer puts things in their mouths gift!
This is one of those projects that I found, pinned and made in less than 30 minutes. I should make a board for that!