It's kind of strange to me to see Futurological Congress and e.g. Fiasco from the other responses being advised as standalone novels (maybe it has something to do with the history of Lem's English translations).

I'd instead advice to read the whole Ijon Tichy cycle starting from Star Diaries (which are pretty similar to Cyberiad in style), and the whole Pilot Pirx cycle starting from the Pirx Stories.