school buses and fire trucks
Ally has a wonderful game that we play to keep her occupied in the backseat on the way home from daycare/work. It's called "I found it"

It works like this. Ally leaves daycare and usually sees the preschool bus dropping off some kids at daycare on the way out. She yells I found it or school bus or both and then we get in the car. She asks me where the school bus went and I tell her I don't know, maybe we'll see some more. She asks me to find them, and I tell her I'll try. She then looks out the windows absently until she sees another one. Then she shouts "there it is" and asks me if there are kids on it. She's endlessly fascinated by this game, and I encourage it. It keeps her happy enough that I don't have to turn on one of her CD's.

The variation to this is looking for fire trucks. Backstory: a fire truck came to her daycare a couple months ago, and I assume that they ran the lights and siren for the kids. She was thrilled. Now, however, she is absolutely, irrevocably, convinced that everything with a light and siren is a fire truck. Ambulances, police cars, whatever... basically the game is played the same, but with a lot less successful sightings.