The Advent of Code site features one problem a day from December 1 through December 25. (Actually two, but the problems for any given day are related, the second being a variation or extension of the first.) The problems have cute Santa-related back stories. You can use the language of your choice, so I'm using Haskell.
I have to admit I'm wimping out somewhat, in that so far I haven't written code to parse a file of input data. Instead I've fired up vi and edited it into a list that I can copy, fire up ghci, and say
let input = shift-ctrl-v
and just deal with the guts of the problems. I've not gotten all that far into the problems, but so far, a lot of it has been finding the appropriate library package(s). OTOH, getting to know the libraries is useful; why reinvent the wheel?
Advent of Code is a fun exercise. Check it out.