A future crystal appears in the episode "Dungeon Train" and has the magical ability to let a person see their future by looking into it. It is gained on the Dungeon Train by a random monster as Jake peers into it and sees Finn's future - an old man with armor (and a prosthetic arm) continuing to fight monsters, but with a rather sad emotion. It is used again when Finn looks into it and sees an old Jake following Finn around the Dungeon Train. At the end of the episode, they both look into it after Finn has the will to leave the train, and they both appear in their house washing dishes together.
In "The Hall of Egress", Finn was forced to close his eyes due to the Hall of Egress. The Future Crystal shows Finn and Jake washing up and Finn has his eyes closed. This may have been a view into the distant future around 2 seasons away.