The rest of the codes can be chosen after a theme: fish, pets, foods, plants, etc.
Another method of encoding your communications is to assign alternative "naming" to your map grid.
Instead of using the coordinates (since everyone uses the same map), you could assign letters and numbers or numbers and letters to the grid squares. If you are stuck for which part of the alphabet to use for your letters, you can even pick a word or phrase to use.
If you picked BEDAZZLE and inverted your numbers so they read down the map, your coordinates for each location would be unrecognizable to your opponent.