It depends on the age of your children, but my DS (now 13) and DD (now 10) have enjoyed playing "categories" for several years. We have a fairly broad category (e.g. Disney Characters, Disney Rides, Book Titles, Movie Titles) and then each of us has to think of something in the category that begins wtih "A", then when everyone has finished with "A", with "B", etc. This game is good for longer lines.
When they were in the 3 - 5 yo age range, they liked playing the rhyming game. So, for example, I would say:
"I'm thinking of a word that rhymes with salt -
If I asked you to stop, I might say _______."
They just loved that one, but it requires a fair bit of parental mental effort to come up with 50 or so rhymes in a row.
"Trivia" has always been really popular too, and they actually learn something. We give them everything from math to geography to science questions. "What's the capital of Norway? Name the planets. What is the boiling point of water in degrees Celsius and degrees Farenheit? What's 72 divided by 3?" or whatever is appropriate for the children's ages. In WDW we often add in Disney-themed trivia.
We try to make all of the games cooperative rather than competetive - waiting is hard enough without having them at each other's throats!
Cheers,