If you're talking about any sort of science fiction, I (and I'm sure countless others) would recommend Dune by Frank Herbert -- the best-selling sci-fi novel of all time, and for good reason. It's not as light-hearted as Ready Player One, though.
Just returned the new novella pair Stepping Stone and Love Machine by Walter F Mosely to the public library. I'm a Mosely fan but his science fiction is less conventional than Socrates Fortalow and Easy Rawlins and they're not tight to.the mold of detective fiction. It reminds me of Harlan Ellison a lot and a little of Lovecraft.
My two favorites are Permutation City by Greg Egan and The Golden Age by John C. Wright (actually that one's a trilogy).
I hear good things about Iain M. Banks' Culture novels. I've only read one (The Player of Games) and while it was enjoyable it was still sort of meh, if that makes sense... (I've got Excession in my queue and I suspect I'll enjoy it more.)
While it's more in the realm of science-fantasy, you might enjoy this alternate universe imagining of the Harry Potter story: http://hpmor.com/
Those are definitely two of my favorites. Funnily enough, a few days ago there was a discussion about the most memorable space ships. Mine was "Yggdrasil" :).
What did you like about it? I'd suggest Tad Williams' Otherland, Neal Stephenson's Diamond Age & Snow Crash, and Gibson's Pattern Recognition purely based on subject matter (gamey-cyberpunk-ARG-AI), but none is quite as lighthearted in terms of nodding towards videogame culture.
I too just finished Ready Player One. Such a great read. I tend to lean on Good Reads lately for recommendations. But picked up Atopia Chronicles (http://amzn.com/B00DUK1RKY) after browsing popular sci-fi on amazon's best-sellers list. It has great similarities to Ready Player One - virtual presences, future VR challenges, etc. but a very different kind of book (split into multiple story lines, not one continuous story). But that said, if you've not read Snow Crash then drop everything and read that next!
Lexicon - Max Barry.
The Ocean at the End of the Lane - Neil Gaiman.
Channel Skin - Jeff Noon.
The Long Earth - Terry Pratchett & Stephen Baxter.
The Difference Engine - William Gibson.
It's not easy to recommend something based on a single book but have a look at Vicious by V. E. Schwab, This Perfect Day by Ira Levin or Wool by Hugh Howey.
I run a sci-fi/fantasy blog with a few other people where we review books etc.
So are you looking for more action/adventure type sci-fi, hard sci-fi, space opera, loads of alien life, etc?
I really enjoyed Halting State, and Accelerando, and also the MaddAddam Trilogy by Margaret Atwood as well as Philip K. Dicks works, Valis in particular.
These days I seem to prefer extended series of books that develop a detailed alternate universe. Three truly excellent series (Author-Series-1st Book):
Eric Flint - 1632 - "1632"
David Weber - Honor Harrington - "On Basilisk Station"
Of course, "The Moon Is A Harsh Mistress" if you like it down and dirty, a tale of a "Computerman" working for the man but secretly using his machine to subvert the powers that be up until the final showdown.
I'm ancient and have been reading Sci-Fi my entire life. I just finished "The Darwin Elevator" (1st of 3 in a trilogy) and it was the most refreshing SF I've read in a least 10 years.
