I will give you a use case.
Build Two Applications.
An admin can add products to it and users can buy those products.
When clicking on buy button from e-com app , a REST call must be made. Through that we should reduce the price amount from person’s bank balance. If there’s no enough balance ,we should display insufficient balance in ecom app.
This usecase will help you to get strong in REST calls and error handling.
Happy Learning :)
Please check out the Mendix Academy, they have quite the use cases in there for all kinds of levels: Mendix Academy - Home
Also, the Mendix Solution Gallery can spark your creativity: Low-Code Solution Gallery | Customizable Solutions For Every Industry (mendix.com)