A few weeks ago I was trying to get a few things done before heading out the door and my kids were going a little wild so I quickly put together this little word search for my son and alphabet search for my daughter. It was so easy to put together and it kept them at the table for a few minutes doing something new just long enough for me to gather our things and get ready to go (and still take some photos of them).
I think this would be great while out because you can make endless versions of it on the fly. Just make sure you carry around paper and markers/colored pencils/pens or whatever. You could do this with shapes, numbers, emotion faces, animals, whatever.
The nice thing about doing it yourself is that you can make the letters larger so that it is easier for the younger kids to see and distinguish differences/similarities. It is a great exercise in sight words, learning the alphabet, matching, and concentration.