Toy cars are a good tool to use when introducing counting. Do this informally, during a game. Have a variety of cars out on the floor and ask questions. Try asking "how many cars do I have?" and then count the cars together. Go on to ask questions such as "how many blue cars are there?" and "how many wheels on the car?" Praise success, but do not force children to join in.
Toy cars can be used to develop the idea of subtraction. Again, do this while playing a game and stick to talking; don't write math questions down and expect a preschooler to fill in the answers. Again, ask questions during play. Try "I have four cars. I'm going to give you two of them. How many have I got now"? Then count the cars together.
This activity is best done with a small group of children. Encourage the children to share a number of cars equally among themselves, talking through what they are doing. Talk about how many cars there are -- how many children -- and how many they must each have to make sure it is fair. Then have children check they have the same number of cars. Introduce fraction related math terms by pointing out that four cars shared between two children means they have half of the cars each.
Most children enjoy racing cars against each other and this can provide a good opportunity to introduce and reinforce the concept of number order. Give each child a car to race and talk about whose car came first, second and third. Whose came last? Talking about these ideas when children are playing will help them become familiar with the terms -- and will develop their number knowledge.