This website is seriously addictive: Free Rice. You answer questions about what a word means, and every time you get the answer right, the sponsors of the site donate some rice to the United Nations World Food program. It’s set up to be at exactly the right level for your word knowledge – moving you up if you get 3 words right in a row, and down if you miss 1.
As soon as I showed my parents the whole household became very competitive. My dad got up to Level 49, but couldn’t sustain it. Mr Penguin got to Level 48, and so far I’ve only made it as far as Level 46. Enjoy!