The Futurological Congress is also a lot of fun; it's one of his comedies, a satiric look at futurism.
His Master's Voice is advanced level Lem. It's less a novel than a fictional autobiography of a scientist, and while the subject matter is fascinating, it's a bit dry.
The Cyberiad is a classic, but very different and not really sci-fi. It's a collection of absurd, surreal fairytales about a pair of robots. It contains math puzzles and rhymes and lots of weird stuff.
Fiasco, Memoirs Found in a Bathtub, Tales of Pirx the Pilot and Star Diaries are supposed to be good.
I found Eden (one of his early works) to be a little juvenile and poorly written. It, too, is about first contact gone wrong.
Like Blade Runner's relationship to Do Androids Dream of Electric Sheep, it was a lot different than the book, but shares some deep ideas, and stands on its own as a great movie.
The scene in the USC ICT's motion capture studio was riveting, with Robin Wright playing a partly fictionalized version of herself, and Harvey Keitel playing her agent, baring their souls to the giant emotion capturing machine.
https://en.wikipedia.org/wiki/The_Congress_(2013_film)
As far as I've read, plot has absolutely zero to do with The Futurological Congress. The only similarity is that there's something in the movie called the "Futurological Congress" and that there's some kind of attack on it.
If you think it's worth seeing, maybe I'll give it another go.
It may be an easier entrypoint than Solaris, at cost of arguably not being as "deep" in its reflections.