Go and build a simple Spring Boot microservice that takes in HTTP requests (via REST) and persists them to persistent storage.
This is a classic microservice and you will cover a lot of ground:
SOLID principles
Inversion of Control/Event driven
Composition vs Inheritence
DAO implementation (JPA/JDBC/take your pick!)
Transactions
Idempotency and REST
Controller/Service/Repo layers
Unit tests
I say to use Spring Boot to do this because it abstract a lot of the implementation away and allows you to focus on coming up to speed with enterprise design patterns and architecture.
This tutorial is from: https://spring.io/guides/gs/spring-boot/