I subscribe to the Word of the Day from this site. I don't often learn brand new words, but I enjoy hearing about related words in English, how the word evolved to its current meaning, or even tips on how to remember to spell them.
I saw this, specifically designed for English learners, and thought I'd share.
Getting the Hang of Idioms : Weekly Worksheet : Thinkmap Visual Thesaurus
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.