Research for video game: Novice cryptography concepts
Depending on your video game design you maybe can utilise a many-time pad. Many-time pad is many plaintexts encrypted by XORing them with a common string. 

One-time pad cannot be broken but many-time pad can, especially if the players can make an educated guess about what the plaintext message might contain.

You can also try encrypting numbers with RSA but with small modulo n so that players can easily factor it.