Wish I had left home: my debit card. Wish I had packed: a pre-paid debit card from my bank. This worked out great when I went to Disneyland last summer because I knew exactly how much I wanted to spend and wasn't planning on charging anything other than what was already paid for. I'll never pack my debit card again (even for emergencies) because the last time I was at WDW it got lost somewhere between the Chinese Pavilion one night and the Tower of Terror the next morning. I never retrieved it. Lost forever in the Twilight Zone. Tip: Pack a disposable camera. Some places do still get them developed and you never know when your good camera might malfunction or accidentally crack while you're on a ride. If you don't use it then you have your backup for the next trip or family function.