In the training courses, there are multiple ready made apps that you could use for testing.
From the intermediate learning path I would suggest Advance Page Building which demoes multilingual expense reporting.
Building an Event App Using Microflows is also nice, but it required Mendix 9.6.1, so it is kindof outdated.
Also. you can download complete apps from the marketplace:
You can simply create a demo application yourself, and host it as a free application if you don’t want to use any of your paid nodes.
This will have the advantage of you having full control of the application so you can write your own functionality to test with ATS without being restricted to how an existing application works.