I numbered each compartment of the egg carton 1-12 with a permanent marker. Then I found lids (mine happened to be from baby food pouches. Milk lids, juice lids, or pop lids would also work great) and numbered them 1-12.
And there we had an easy learning activity that didn't cost anything we hadn't already spent and took less than 5 minutes to assemble. He was able to start matching the numbers on the lids to the numbers in the egg cartons. Eventually, I would like to move on to counting beads or beans to put in each compartment.