I am fairly sure a recommendation to this website
has appeared here before but just a reminder....
You answer questions of increasing difficulty on a variety of subjects, and grains of rice get donated to The World Food Programme combating hunger. You can even set your own level - the English vocab category starts with very simple words but it soon escalates. I couldn't get further than level 54 of 60 because you get knocked back a couple of levels for incorrect answers.
The nice thing is that it makes you learn new words because it gives you the correct answers as you go, and then the same words resurface again.
Prepare to spend many happy hours. I am off to try another subject