In Strata from Terry Pratchett, a 24000 km flat disk world, complete with mechanically animated celestial sphere is planted in our universe (something like the Earth as it was seen in medieval age, i.e. flat, and you could actually fall off if you came too close from the edge).

Also some peoples make jokes like planting dinosaurs fossils wearing wristwatches, or holding a "No to nuclear power" signs.

If I was doing it, I'd plant a huge perfectly black monolith (something like the 2001 one, but bigger) with on the middle of one of it's faces a small push button and a "Do not press" sign.

In Strata from Terry Pratchett, a 24000 km flat disk world, complete with mechanically animated celestial sphere is planted in our universe (something like the Earth as it was seen in medieval age, i.e. flat, and you could actually fall off if you came too close from the edge).

Also some peoples make jokes like planting dinosaurs fossils wearing wristwatches, or holding a "No to nuclear power" signs.

If I was doing it, I'd plant a huge perfectly black monolith (something like the 2001 one, but bigger) with on the middle of one of its faces a small push button and a "Do not press" sign.