The matter of the diversity of rewards.
Freerice needs more rewards while you grind. Like a Title system, take for example: Multiplication Table: Title 1: Req: 100 grains Title Name: Multiplication Table Tester I.
And you can choose any title you want to display below your Profile Picture.
Or you could just use simple levels to display. Another idea for rewards is Certificates like "Vocabulary Pro/Graduation"
Yes! Mafia Wars does this exact thing! You get titles for beating certain places, and can choose your title!
I agree, it's a simple strategy used by most every game that has any level of addiction, such as MMOs (like WoW or Everquest), and First person shooters (such as Call of Duty or Gears of War), and many other games. It gives the player a sense of achievement and pride.
This will give users and me have a greater sense of reward. If this is implemented I think much more users would stay longer.