Good Luck Viorel!
I don't know what Mansystems wants specifically, but if you want to demonstrate skill with the Mendix platform it is a good idea to include a web service call. Maybe incorporate a REST service into your sample application as outlined here. I'd also recommend a few more complex microflows beyond creating and committing entities. Maybe you could have some logic to check availability of ski equipment at rental time against the amount of items in inventory. Maybe you could have the app send out confirmation or receipt e-mails using an SMTP e-mail relay. It's pretty straightforward to set up using a free gmail account and it might show off your skills.
Another thing you can do is create a custom theme. A good looking application can go a long way, and it will also demonstrate your UI/UX skills.
Make your own demo system containing the best showcase apps that you have built when following along with tutorials. Especially with some app that has your interest.