Basic UI app
First create an app which just has a basic UI. A one page app. This way you will learn how to add UI components like images and text to your app.

Multiscreen app
Then create an app which has multiple screens. This way you will become familiar with navigation.

User input app
Create an app which takes input from the user and uses that input to show some message to the user on the screen. This way you will learn how to get input from the user and how to store and use that input.

API app
Create an app which uses an API. This way you will learn how to make API calls. This is a very useful skill.

DB app
Create an app which stores, updates and reads information from a database. This way you will learn how to perform CRUD operations in your app.

Firebase app
Create an app which uses firebase. This way you will learn how to use the various features available in Firebase such as Firebase real-time database, etc.