Think about something that would be useful for you and bring you value. That will be something you enjoy building most.
For the portfolio portion maybe tie in something that would be valuable to your typical client.
That said here are some ideas:
Simple budget app to track/project your spending/decide on purchases/project income?
Track something you collect. Lego minifigures maybe or episodes you've watched of your favorite tv show.
Budget/Time estimating tool for work projects.
A real time chat app or social media app for you and your family and/or friends.
A 'dashboard' app for you and your family, share your locations, leave messages, simple to do lists, weather tie in some apis of services you all use.
Not sure what framework you use or are planning on learning.
I've been using Laravel for a couple of years now and have been using Vue js more and more. They are a great combo and auth comes together pretty much right out of the box.
https://laracasts.com/series/whats-new-in-laravel-5-2
Laravel has some great tools
Great documentation: laravel.com
local development made easy: Homestead (Virtual Box) and Valet (a new tool for OSX that creates a lightweight server that would be perfect for your project).
Build Servers and Deployment: forge.laravel.com (setups SSLs from dashboard through LetsEncrypt).
Envoyer if you need zero down time deployment.
Nice video tutorials at Laracasts.com
Enjoy your summer vacation and good luck building a cool app.