An internship will include a series of mini-projects for your intern. You want to introduce them to as much of the following body of concepts throughout these mini-projects as possible:

• Testing
• Documentation
• Package Dependencies
• Object-Oriented Inheritance
• Code Maintainability and Technical Debt
• Complex Version Control and Branching Strategies
• Scaling and Distributed Systems
• Continuous Integration and Continuous Deployment (CI/CD)
• Monitoring, Logging, and Observability
• Security Practices and Threat Modeling
• Cross-Team Collaboration and Communication

Having said that, good first tasks involve getting a high level grasp of the open source project, maybe getting a working development environment, running a build, going through the tests.

You want to gradually increase complexity and the areas in which the intern is active, besides having pull requests merged in the open source project of choice, you want the ultimate tasks to allow them to demonstrate meaningful work which they could discuss in a job interview.