A lot of good answers above, I just wanted to add...
Tips. In fact, keep a wad of singles handy. Maids, valets, bell hops, bribes to keep the kids quiet...
"Anybody have room for desert?" I never remember to budget for deserts or special drinks.
"Oooohhh, look at that! I want one!" and remember, you may be saying this yourself, it's not just the kids that want to buy every thing they see in a store....or on a snack cart!
"oops, I forgot to pack..." You always forget something...well, I do, anyway.
...and money to buy an extra suitcase to carry home the 900 pounds of stuff you bought (or is that just me?).
...and money to pay the airline when your checked bags are over weight (me again?)