What new and exciting things can you do with the things you see while you're traveling? Turn them into sentences, of course.
Give your kids a time limit -- say, five minutes -- and let them use that time to "collect" words they see out their windows. These can include the names of objects, descriptions of the items they see and hear (such as "blue" for the sky, "tall" for skyscrapers, "rumbling" for motors, and so on) and actual words your kids cull from signs. Your kids can choose whether to list the words they find on paper or to simply remember them. When you call "time's up," your children use the words they've collected to create as many sentences -- serious or zany -- as they can.
Your family can work as a team to create as many sentences as possible. Alternatively, you can ask kids to see how many sentences they can build by themselves, using either words they've collected on their own or taken from the family's pool of words. Your children's goal then might be to beat their own "personal best" number of sentences from the previous round.
So perhaps now is a good time to practice collecting a sentence. Look out the window. What do you see? -- a tractor, a cow, farm hands, and flower beds. Well, how's this for a silly sentence? "The farm hands put the tractor to bed and the cow covered it with flowers!"