Computer related technical skills are usually thought as complicated and difficult to understand. It's very difficult for one to get hands on one skill or master one skill. But if you really do want to learn something useful within one day, there are some good choices which will not take too long to get to know and use..
Version control:-Git, GitHub and SVN
Regular expressions
AWK
sed
Grep
Learn how to do things with Vim that you never knew could be done.
Set up a crawler that can scrape some webpages and parse some basic data.
Set up a bigger crawler that has to fill out a form or two.
Program a basic linear algebra library (matrices, vectors, multiplication)
Add least squares regression to this library.
Make your library work efficiently with sparse data.
Learn how to use list comprehensions in Python.
Get a Stack Overflow account and learn to use the site.
Read the freaking manual for your favorite language.
Implement a simple machine learning algorithm on your own, with a whole pipeline.
Learn the how to make a simple line graph in Excel.
Get your eclipse installation fully pumped up.
Learn the basic functionality of a NoSQL database.
Learn the most basic functionality of SQL
Understand difference between SQL and NoSQL databases (strengths, weakness, limitations, where to use which and why etc.)
Getting comfortable with Linux.
One or two sorting algorithms. (Perhaps Quicksort and Mergesort)
Learn how to effectively develop unit tests for your code.
Familiarize yourself with some of the AWS services and their API in the language of your choice
One algorithm a day
Understand need for distributed processing and distributed data storage and challenges in them (basics of CAP Theorem, MapReduce algorythm, clustered MySQL or PostgreSQL database)
Learn how to edit Wikipedia articles both syntactically and under wikimedia guidelines, such as neutral point of view.
Learn how to write in markdown
LaTeX, BibTex, and pgfplots
Learn how to work from the command line
Learn JavaScript
If familiar with OOP, learn design patterns.
Ok, why not go and have a try?