I cut some of my brightly coloured 12x12 card stock into squares, and wrote a number from 1-20 on each one. Then I used my basic single-hole-punch and punched the corresponding number of holes around the edges, maxing out at 5 holes per side. These took me about 10 minutes to make, start to finish.
Then I turned them all face down, and I would call out a number, like "Find the five!" and my son would look at the dots and try to guess which card would have a number 5 on the back. For the first few times we played this, I only used the 1-10 cards, but once he got really good at those I added the next five, and finally worked our way up to twenty.
It is fun, and he is actually getting pretty good at finding the right cards without having to count each individual hole in the card before he flips it over. Look how excited he is when he gets it right!
Another activity option...
We have also pulled out some shoelaces (dollar store has packs of 10 for a buck) and turned these into lacing cards.
For an older child, any card over #5 could be used to learn simple addition equations. Have them face down, and show the child the dots on the sides. If there are five dots on this side, and four dots on this side, how many all together? Then they can check their answer by flipping over the card.