It's never too early to start working with your children on counting and grouping, and you can use just about anything. I'm partial to using a deck of cards, though, because they are so simple to use for this and so many other activities as your child gets older. This basic activity is designed for young children up to the Kindergarten level.
Give the child the entire deck (minus jokers, rules for Texas Hold 'Em, and other extraneous cards). Ask them to group by color, suit and number or letter (rank). Make sure you encourage them to arrange all of the cards into their respective groups. As they arrange the cards, have them count how many are in each group, and ask about whether the groups are the same size or not.
You might explore this idea visually, having them arrange groups into equally sized rows and columns. I've seen other teachers have students stack the groups of cards vertically and talk about the relative heights of the stacks.
The possibilities for extending this simple exercise are endless: you could have them group all of the cards bigger or smaller than a certain given number, or shuffle the deck and challenge them to create groups more and more quickly. You could play any number of traditional card games that covertly teach math skills.
You could even explore ideas such as even and odd numbers, but at some point you're going to get into concepts that are far beyond the Pre-K/K level. Then again, if your child seems to like this kind of game and is progressing quickly, you should by all means raise the level of difficulty as high as you can.
All in all, you can do this anywhere with no setup, and in a short period of time. If you're worried about your kids ending up in the World Series of Poker due to their use of regular cards, you can do a lot of these activity with a deck of UNO cards (not to mention by playing the game itself).