Sticker books, magnet books, coloring books.
I second the idea of a travel magna doodle. They have been lifesavers for us on long car rides or on a plane.
Playdough. Finger puppets. A special snack (depending on what time of day it is).
This may not work if other people are in the lobby, but my 3 year old's favorite activity to do lately is hiding and finding easter eggs. I bought a bunch of plastic eggs on sale after Easter, but you could hide just about anything - small cars, etc. Along the same lines, play I Spy (something red, something that's a circle, something that begins withthe letter B).
I really like the idea of singing action songs. Keep a list of various songs to sing with you. (I know that my mind always goes balnk in the moment.)