1. Euphemisms in English and their meaning
2. Cultural references in the language and the literature behind them, such as Shakespeare's "There Is a Tide". A great list of this is in E.D. Hirsch Jr.'s book "Cultural Literacy".
3. Words from English that have been adopted by other nations, like people in Iran using taxi and hi.
4. The languages that have contributed to English, such as French loan words, Greek and Latin terms used by English
5. Why English lost its gendered pronouns (due to Scandinavian invasion)