You may want to let them try and work out how long it will take you to travel somewhere when you get in the car, by telling them what speed you'll be traveling and how far away your destination is.
Combine math with reading _ There are quite a few nice children's books out there about math and who does not like a deal. Imagine it's a 2 for 1 deal you are getting, with the opportunity for your kid to enjoy reading and math at the same time. The Sir Cumference series comes to mind with its attractive illustrations and adventures. Less than Zero is another. It gives a good introduction to the concept of negative numbers. Also One Hundred Hungry Ants appeals to kids too. Well who doesn't love bugs? Check out online stores where you can read reviews but remember to ensure that the reading level is not more advanced that necessary to get the math concepts across. If that is the case that will just take the fun out of it and make your children frustrated.