This is Lem in his high comic mode. If The Cyberiad is a collection of SF folktales, The Star Diaries is a collection of SF tall tales, fish stories from outer space. Some are just light entertainment; some are satirical fables; all are brilliant. Most of them were written in the fifties, but some appeared later, as described in translator Kandel's afterword.
They're all stories about (and told by) Lem's perennial comic hero, Ijon Tichy. He is a cheerful, nonchalant space pilot who is fond of hanging his wet spacesuit out the porthole to dry. He's seemingly capable of surviving the most astounding ordeals, from being disintegrated into his constituent elements to being endlessly reduplicated in a time loop, so that he ends up arguing on Friday with his previous self from Thursday about which one of them gets the spacesuit that he needs to wear on Saturday, not to mention which day of the week stole the chocolate.
In another story, he travels to the 27th century (recruited by his future self, somewhat in the manner of Heinlein's "All You Zombies") to take control of a history-repair agency, whose time-traveling agents inadvertently cause all of the disasters in human history. Larger accidents result in the asteroid belt and the craters on the Moon.
In the most overtly satirical story of the lot, Tichy has to escape from a planet whose government has legislated that all of its inhabitants shall henceforth breathe underwater. The citizens sing patriotic anthems about fish and humidity, and learn that in a future paradise all shall become "gwats" and "sunkers," idealized water-breathing forms. Debates rage over whether gurgling is allowed. It amazes me that that one got published, given the place and the era (Poland in the 1950s). The longest and latest-written is a surprisingly somber meditation on the social dangers of unlimited bodily self-modification.
Tichy's good friend, the prolific inventor and astrozoologist Prof. Tarantoga (who, "out of concern for our youngest citizens, whom parents on occasion do leave in the house alone...devised lighters that will not light" which "are now mass-produced on Earth,") supplies two introductions. Lem illustrates some of the stories with amusing pen-and-ink doodles, some of which reveal his training in skeletal anatomy. My favorite illustration shows a tiny planet festooned with signs reading DON'T LEAN OUT in four languages, like all of the trains in Europe.
Tichy's Eleventh Voyage provided loose inspiration for an early episode of Futurama, "Fear of a Bot Planet". (I'd always figured as much, but writer and executive producer David Cohen confirmed it on the season 1 DVD.)