If you haven’t already seen this, you might like to give it a go — not just because of the free rice aspect, but because it’s a fun and well-designed game.
Get three in a row right and you go up a level — get one wrong and you go down a level. At higher levels you get asked more difficult words, difficulty being assessed by the proportion of previous contestants who’ve got them wrong.
50 is the highest level, so if you can stay there for any length of time, you’re doing pretty well. My longest run at 50 was 4 words. Beat that if you dare!
And you’ll learn lots of interesting (but, for the most part, practically useless) words.
(Thanks to watervole for pointing me at it!)