The Eyes of the Overworld (or perhaps anything that grants astral vision) allows you to see anything invisible within the astral vision radius.

Potion of astral vision, which confers temporary astral vision. To give this some use, also add some sort of being that appears on the Astral Plane and is only visible with astral vision.

Artifact blindfold The Double Eyepatches, which has the normal effects of a blindfold plus radius 2 astral vision.

If you fall asleep while having astral vision, you get astral projection. This consists of you spawning as some sort of incorporeal monster which can phase next to your body. Your body can’t move. Your projection can use your items like normal (consumables and charges are used up). If your projection dies, you return to your body and sleep out the remaining duration. If you wake up, your projection is destroyed and you return to your body normally; however, combat will not awaken you. If something else kills your body while you’re projected, you die and the game ends.