A great thing about Lem is that he has written in (the style of) so many genres. The third column in this table on Polish wiki illustrates this rather well: https://pl.wikipedia.org/wiki/Lista_pierwszych_wyda%C5%84_dz...

There's sci-fi of course, but also crime fiction, "realistic" fiction, philosophy essays. Further, something not easily gleaned from the table is that his sci-fi can be humorous, horror-y, thriller-y, book-for-kids-y, etc.