--Some problems you could try--

1. Math:
Print the sum of the first 2000 Fibonacci numbers 
Print angle given 2 intersecting lines

2. Physics:
Print max load of a crane given counter weight's mass and arm length
Print horizontal distance projectile will travel given initial speed and launch angle.

3. Rendering:
Draw a rotating cube
Draw the mandelbrot fractal 

4. Game:
Flappy bird copy
A first person shooter
Car driving game

5. App:
Play notifications sounds of common apps every x seconds