Make a game using UIKit, and learn about integers, buttons, colors and actions.
In this project you'll produce a game that shows some random flags to users and asks them to choose which one belongs to a particular country. After the behemoth that was the introductory project, this one will look quite easy in comparison – you've already learned about things like outlets, image views, arrays and Auto Layout, after all.
(PS: If you skipped project 1 thinking it would all be about history or some other tedium, you were wrong. This project will be very hard if you haven't completed project 1!)
However, one of the keys to learning is to use what you've learned several times over in various ways, so that your new knowledge really sinks in. The purpose of this project is to do exactly that: it's not complicated, it's about giving you the chance to use the things you just learned so that you really start to internalize it all.