Not sure about the length, but Vernor Vinge's Rainbows End is one of my favorite works of speculative fiction that really aged well so far.

"Short enough to finish in a day" seems pretty tough for me, but maybe I read slowly.

Short story books are good for casual reading in short sessions. Robot Dreams by Asimov, or Welcome to the Monkey House by Vonnegut. I used to carry each of those around and read a short story while waiting at a restaurant or at the DMV or whatever.

I really liked Altered Carbon. Approachable sci fi with drugs, violence, sex, politics, and of course high tech ideas like flying cars, AI hotels, digital consciousness.

The Lady Astronaut series by Mary Robinette Kowal. The first book is called The Calculating Stars. Basically, an alternate history where (spoiler for the opening chapter)

spoiler

a meteor wipes out the east coast and kick-starts climate change, causing the Space Age to start 10 years early.

It follows a Jewish computer (a woman who literally runs calculations for NASA, as seen in Hidden Figures) who wants to become an astronaut, and her struggles with the racism and misogyny of the 1950s.

I would recommend The Wheel of time, but be aware that its a very long book(series?). It contains 14 Books and totals at about 11k Sites. It absolutely takes quite a lot of time to fully read it, but its absolutely worth it. Its by far the best book ive read so far.

Not sure if I'd say they hit hard, but for readability it's hard to beat Rex Stout's Nero Wolfe books. Some of the best murder mysteries I've ever read, so much fun.

The Heart Shaped Box

NOS4A2

Between Two Fires

The Troop

The Princess Bride

Edit: Just realized you're looking for something to finish in a day, my bad. Have your read any Sherlock Holmes? They're entertaining and you can get through quite a few stories in a day.

A book that stuck with me for a long time was The Midnight Library by Matt Haig. It was a fairly quick read too, I'm a slow reader so definitely longer than a day but I think I read it over a short vacation.

The Handmaid's Tale

We

Nightfall

The Terminal Experiment

Raymond Chandler's novels, esp The Lady in the Lake

The Pirx the Pilot stories, 8 in 2 volumes

Dashiell Hammett's The Maltese Falcon and The Glass Key

2nd the Hitchhiker's Guide and they're easy to rejoin

A A Fair's novels are short and have odd western us lore in them, one has a great way to bet in Vegas, others name spots in Mexico, they were Gardner's fun books that he liked to write more than other series.

I read a lot of thriller/mystery books. Almost anything by Frieda McFadden is a great read and are quick weekend reads.

Read the short story : the approach to al-mutasim By Jorge L. Borges: https://ia601405.us.archive.org/10/items/HeliganSecretsOfTheLostGardens/BorgesJorgeLuis-TheApproachToAl-mutasim.pdf#:~:text=The%20Approach%20to%20Al-Mu'tasim%20Philip%20Guedalla%20writes%20that

If you like it, and like shorts stories, Borges will also become your favorite author. A good start is :

A Universal History of Infamy: A Universal History of Infamy Giving some more or less accurate retelling of the life of evil people like Billy the kid.

Lots of great recommendations here. I'd also add Curse of Chalion by Lois McMaster Bujold. Her Penric novels are quite fun, too.