Anytime, anywhere, anything can turn into math. In the car, take turns asking each other problems (let the kids pose problems too!). Start with concrete scenarios: "So, if we baked 100 cookies, and there were 5 of us, how many cookies would we each get?". Or if the kids are into fractions, make it 17 cookies. It is easy to ask a hard problem, as you may find out when your kids take a turn to ask you!
Try to ask problems that are a little bit hard for your kids, but not way above their level. Then, if the don't get the answer right, DON'T TELL them how to do it. Ask an easier or more concrete question instead, until they can get it right. Try to ask related questions that may help with the first one.