My kids...well as of this point, I know one is wearing a Pikachu costume because it is warm. It's August. Another claims playing Civilization VI will help with AP history. A third explained he's very good at creating art in Minecraft. I'm not sure what skills that might translate to in the non pixelated world, but his siblings are impressed. Ninety percent of parenting is playing the game, "What's my motivation?" so I've compiled some analogies from popular culture to help motivate all my kids to get ready for school again, using cool geeky references like all the kids do these day. Failing that, I will speak in text, and that should do it.
You should get your summer work done because....why?...well consider the alternative.
The Pokemon Quest, Will You Please Go Now?
Son. You want to be the very best, like no one every was. On some campus somewhere, are Pokemon for you to catch with your phone. Stay here, and your dad and I will cite Team Rocket's motto to you night and day. Here's the link for the common ap. Surrender now...
So...to put another way, reaching into your own family lore: "Now is the time to start the big push."