Well, the last two are just to make it simpler for you to do.

As for ideas - you first have to tell us what you've learned about C# so far. What concepts are you familiar with? Basic syntax? Classes, objects, composition, inheritance? Value types / reference types? Private / protected / public members? Fields, properties, methods? Static memebers? Conditional statements (if, if-else, switch...)? Loops (for, foreach, while)? Generics (like List<T>)? Events? What parts of the framework library you're familiar with? Have you programmed WinForms before? Console applications only?

Also, do you have some additional broad guidelines and constraints? Any ideas so far? What kind of application is expected of you, more specifically? Maybe a simplified mockup of some business app or tool? A (simple) line drawing application (there's an idea for you - and one that's not too hard to implement)? Elaborate a bit more.