Cyberiad or Star Diaries are good starts for the humorous (but still philosophical) stuff. Solaris is his best known work, serious, and excellent. It’s tricky because Lem is almost two authors, one serious and one ironic leading to zany. Everything is short, so easy to just try.

Yeah. I would also recommend the Cyberiad and Star Diaries to start. Both are slightly interconnected short story collections with a lot of humor. The Futurological Congress is a complete (but short) novel and the first follow-up to the Star Diaries, it's also a good place to start.

For Solaris (a serious novel) I would recommend the new Kindle translation, if you are reading in English. The old one was a retranslation from French for some reason.

Pirx the pilot

And whatever you select otherwise, definitely be sure to not miss Lem's "Golem XIV": Lectures by what nowadays is called an AGI to Humanity, short before that entity evolves to an even more advanced level - from which further communication with humans will no longer be possible, and that likely operates outside the physics known to us. Stunning, especially when considering it was written in 1973-1980. Seemingly, the English translation of Imaginary Magnitude contains Golem XIV in its entirety; in German and other languages it was published as a separate book.

what a great book is Golem XIV!

the concept is cool, but the real gem is the fact that it's not really written in the form of a novel. And yet, it's still recognizable as one. I love that Lem explored in bending these definitions

I read Solaris and Cyberiad back to back without realizing they were by the same author. I'd recommend both.

I'm not taking issue with the other recommendations, they're excellent, just adding this one, from the non-humorous hard sci-fi side of Lem: The Invincible. Really short, really tight, some very hard sci-fi concepts being explored, still works as a story. A bit spooky.

I'm not into Lem's humorous stuff, but from his serious books I would recommend to read Solaris, The Invincible, His Master's Voice and Fiasco.