Those projects that you send as a portfolio have to include:
- A project showing that you know how to use basic CRUD operations
- That you know how to use the most essential functionalities of a certain framework (usually the one you will be working in the job you are applying for)
- An app that works on different platforms (mobile/web/desktop)
- Apps that use third party APIs or libraries
- An app that is already deployed somewhere and working
- Apps that use file I/O
Here are some of the best examples for applications to use in your interview portfolio:
A blog or a forum-like web app
An application which implements content posting and social interaction, you can show your abilities to develop programs that make the basic CRUD operations, are served on a web server, and also have some good UI/UX design. You can also add some more advanced things like user authentication, search, caching etc.
Skills you can show: Database, HTTP server, HTML/CSS
Expansion options: User login, frontend editing, build an API, search, link analysis

An e-commerce app
An E-Commerce application is also one of the most basic scholar examples of a portfolio app, but a good one to show your skills. Here you can show your dynamic content generating skills, as well as UI/UX design and some more complicated flows. You can even integrate a real-life payment solution, like for example PayPal, Payoneer, Stripe etc.
Skills you can show: Database, HTTP server, HTML/CSS, Dynamic content building
Expansion options: User login, payment integration, e-mail sending