I attended the FOSSASIA at Singapore on 13th-15th March 2015. I also bring the HTML Puzzle Box to the event. Many attendees find it interesting to accept the #HTMLPuzzleBoxChallenge to build the right structure of HTML tags in less than 30 seconds. Of course it's easy to do basic HTML, but many people find it interesting to experience the physical boxes of HTML tags. And some people found it confusing to build the boxes. I built the idea of HTML Puzzle Box using papercraft, so it'll be easy for everyone to build their own, and try this with everyone.
HTML Puzzle Box is a free to use, you can find download link from here.
This is an idea i've submitted to Mozilla Webmaker project as No-Fi teaching kit. So you can help to teach HTML without needing any internet connection. Please visit webmaker.org for more informations.