I was going to suggest the same (Project Euler specifically). I think the great thing about a site like Project Euler is you have a set result your code must p

I was going to suggest the same (Project Euler specifically). I think the great thing about a site like Project Euler is you have a set result your code must produce to get it correct, so you must think through the correct way to code the algorithm. Another benefit of Project Euler is, after you solve the problem, you can discuss different approaches to the problem in different languages with other people who have solved it. Reviewing these discussions can help to see if there are other ways to code the program which may not have occurred to you.