a somewhat serious answer: simple things should be easy to you, and hard things possible examples of simple things: DFS/BFS walks; simple Project Euler problem

a somewhat serious answer: simple things should be easy to you, and hard things possible examples of simple things: DFS/BFS walks; simple Project Euler problems; or "write a simple game in terminal, maybe with some form of minimax search" (a bit harder), or maybe a parser for simple arithmetic expressions