Read a little about how to read a lot more in the future.
Go to https://www.ixl.com/signin/hoovercs, log in, and learn some language stuff.
Go to vocabulary.com, log in, and increase your vocabulary
Go to the Method Test Prep site and practice an English related skill (English test, reading comprehension, analogies).
Practice your typing here: http://typingclub.com/typing-qwerty-en.html
Go to Grammar Monster, read and learn about one of the topics there (semicolons, conjunctions, adverbs, apostrophes, etc.) and take the tests available on those pages
Play Verb Viper
Play The Grammar Gorillas
Learn something new by reading an article at Wonderopolis
Become a Google expert and learn how to search more effectively when you're looking for information: here and here and here
Test your Google research skills by playing the Google-a-Day game
Take one of the grammar tests on this Grammar Monster page and see how you do (and then see Mr. Lochamy about any questions you miss that you don't understand)
Go to chompchomp.com, click on one of the topics, read all about it, learn something new about grammar and writing, and become a smarter person
Look through the Common Writing Mistakes Made by Students document and learn a thing or two (or, at the very least, make sure you understand and are avoiding these mistakes)
Become a rhetoric master by learning these advanced devices for using language creatively and effectively
Play this game and learn an important thing or two about how to think and solve problems (and not be too quick to trust yourself) correctly.
Go learn some interesting facts about the origins of numerous words: Word Origins with Dr. Jacques Bailly
Go here to see what mental_floss can teach you about a cool language trick: the paraprosdokian.
Go learn about these common language/vocabulary mistakes made by smart people.
Watch this video (even if you've seen it before) and check to make sure you have the proper approach to learning (and life): the growth mindset
Learn some interesting language tips and tricks from Grammar Girl.
Think proofreading isn't important? Check out this mental_floss article detailing just how pricey typos can be.
Go learn a new language using Duolingo (as studying a different language is a great way to make you think more critically about our own and how it works).
Take this quiz to find out how highbrow you are. Then become more highbrow by signing up for the Highbrow daily e-mail service.
Learn all about eggcorns and avoid making these common mistakes.
When you think, do you think about these cognitive biases (and make attempts to keep them from clouding your judgment)?
Go spend some time at freerice.com and practice all sorts of skills while helping to provide food for hungry people.
Get out an interesting book and read it.
Go to sporcle.com and find some quizzes that will challenge you (like ones that touch on academic related things).
Work on a Washington Post crossword puzzle.
Test your copy-editing skills with these cool quizzes from The New York Times: Quiz 1, Quiz 2, Quiz 3, Quiz 4, Quiz 5, Quiz 6, Quiz 7, Quiz 8, Quiz 9
Learn a thing or two about how to (or not to) argue.
Watch an episode of Words Unravelled.
Learn about interesting etymology with linguist Gareth Roberts.