Temples with "magical", but never too harmful artifacts, and statues of "gods" everywhere, along with some sorts of scriptures future civilisations will be easily able to decode. These things would be relatively hidden like easter eggs, only to be found with the slightest research or be stumbled upon, and the scriptures, along with the temples and statues, would have as purpose initially confusing the new civilisations, and eventually guiding them into starting a religion which makes absolutely no sense.
With completely stupid rituals featuring stuff like, say, everyone spinning around, and singing 'trololololo' with rubber chickens on their heads as an attempt of making crops grow faster.
I know I'm not thinking too big here, but it sounds like trolling nevertheless.