Advent of Code[1] - A set of 50 tasks, released annually, with increasing difficulty. Each task is a short, Christmas related, funny story which makes it easier to follow and complete. You can track your progress so it'll give a sense of completion and satisfaction. The drawback may be that challenges are language agnostic so they aren't crafted for learning specific features of any language.

[1] https://adventofcode.com/2017/events (3 editions has been released so far)