Dubloons
Since 1 dubloon is worth more than 2 dubloon, you can go on krawk island and use a 2 dubloon coin and eat food worth 1 dubloon, get 1 dubloon change, and make money, heh.
However, you cannot earn points using anything higher than a 2 dubloon, and buying a 1 dubloon food. I tried spending a 5 dubloon coin on a 1 dubloon item, and all I got back was a 1 dubloon coin. I spent a 10 dubloon coin on a 1 dubloon item, and got back a 5 dubloon coin and a 1 dubloon coin. I spent a 20 dubloon coin on a 1 dubloon item and got back a 10 dubloon coin and a 1 dubloon coin. See a pattern?
Also, you can combine dubloons. Usually this will lose you money, but sometimes it will help. Check the wiz to be sure. You can combine two 10 dubloon coins, to make one 20 dubloon coin, at the dubloon-o-matic, below:
http://www.neopets.com/pirates/dubloonomatic.phtml
At the time I made this, the tens go for like 16800, the twenties like 34400. you won't make a killing, but a few hundred here and there.