I've been enjoying the Free Rice website a lot lately. Lots of fun brain-teasers, but I especially like the vocabulary quizzes. Check it out: you can learn new words, and for every answer you get right, they donate 20 grains of rice to the United Nations World Food Program! (Note: the vocabulary quiz is somewhat addictive. As you answer more questions correctly, your "level" increases and the words get harder. If you miss a word, pay attention to the correct answer they give you, as you'll get a second chance on down the line when that word recirculates back to your quiz!)
On the topic of vocabulary building: I just found this enjoyable vocabulary quiz created from C.S. Lewis' The Horse and His Boy. The author of the article hasn't provided answers yet, but I'm guessing you can check back to find them later. Or maybe he just wants us to look up the ones we're not sure about. I'm fairly certain I got most of these, but it made me wonder...how many of them did I actually learn from context, and from reading Lewis? A great vocabulary builder for a young Lewis fan.