Tower Defense with a twist: all towers must be positioned with CSS Flexbox
Another great game to get the ropes of flexbox is Flexbox Zombies (<a href="https://mastery.games/flexboxzombies/" rel="nofollow">https://mastery.games/flexboxzombies/</a>). 
It goes all the way from very basic stuff to things most people never bother to learn, like flex-basis and align-content.