I will second the first part of this - algorithms and data structures. You have to hit the middle ground for those as the language may provide some. So try impl

I will second the first part of this - algorithms and data structures. You have to hit the middle ground for those as the language may provide some. So try implementing binary search trees or splay trees and some graph algorithms. You should not pick too many - perhaps 2 from each genre. At some point you will put these behind you, use pre-written libraries and graduate to an application.