At some point I found Charles Stross' Accelerando, which is freely available online (along with a bunch of other short stories), and found everything I've read (which is most of it at this point, I think) enjoyably fast-paced. In particular the Laundry series (of which I've bought every iteration as soon as they came out over the past few years) -- though arguably these maybe cross-over from SF into fantasy a bit -- and the Halting State trilogy.
In terms of online available work, Cory Doctorow's work is also great. Like Stross, most of his work is more near-term SF, which I have liked, and a bunch of it is available online under a Creative Commons license. It often moves fast and is also politically interesting; sometimes the activism shines through a little blatantly, but the themes are always make me thing in various ways I appreciate (but maybe will fail your lighthearted criterion even if it is fun). His latest is Walkaway, which I would recommend.
I got into Neal Stephenson through Cryptonomicon, which is one of the greats. Stephenson has a pretty rambly, wordy style, but at least that also makes his stories last a bit longer. After that, I mostly moved on to Anathem, Reamde and Seveneves all of which are big and interesting and great reads (it took me longer than usual to get up to speed on Anathem, but it was very much worth it in the end). I tried one of the more historic ones (I think it was Quicksilver) at some point but haven't finished it. The 2017 Rise and Fall of D.O.D.O. was a bit lighter, but lots of fun, too.
William Gibson has already been mentioned, which I would also recommend. I read the Sprawl trilogy a very long time ago, and it's made a lasting expression. I've started rereading it recently. I also liked the Blue Ant trilogy and The Peripheral, but have so far missed the more recent work.
More in the one-off category: The Sparrow by Mary Doria Russell is one of my all-time favorites, and mixes deep SF with psychology and religion in interesting ways. There's a sequel, but it's not as good. I also liked The Time Traveller's Wife by Audrey Niffenegger, which is also on the softer side. I read Scalzi's Redshirts and liked it (especially if you're familiar with the Star Trek universe), it feels similar to Ready Player One in some ways. Daniel Suarez's (or Leinad Zeraus as he originally published them) Daemon is fun and easy to digest, along with its sequel Freedom. I liked Robert J. Sawyer's The Terminal Experiment, but one person I recommended it too wasn't impressed -- I still think it's fun, if not very deep. I also bought his Factoring Humanity and liked it okay.
Hope that helps. Always interested in more recommendations based on this list!