A night time idea. In the campaign setting I'm preparing for, aliens have been and gone a long, long time ago. Humanity lives and breathes recycled air at the very edge of the solar system; some live in religious seclusion, others mine for rare elements, while some prepare for the eventual journey to the stars.
And then there are the thrillseekers.
Aliens left behind things. There are disc-shaped structures of impervious exotic materials, bigger than any human-made habitat. On an asteroid, while digging for a probable vein of uranium, some miners found a cache of odd, metre-high clear cubes. All floating an inch off the ground, with no clear signs of how they were doing it. (When one was eventually placed in a large enough scanner, then scanner reported that there was nothing there) Then there is the religious order based in an abandoned military communications satellite that teaches true enlightenment can eternal life can only be found through hugging (what one scholar described as) alien skittles.
The order's members have no body hair of any kind, and require no food. They also have horrendous halitosis.
Alien tech is magic: sometimes the effects of objects and items can be guessed, but often the mechanism is completely opaque to even serious researchers. Alien tech has mundane properties as well. A perpetual light source, a sheet of "paper" that negates all wavelengths of the electromagnetic spectrum, a bag of sand that organises when poured into one of seven different configurations.
Alien tech is magic in a magic-free universe: resourceful thrillseekers will find uses or buyers for what they find. It's not the aim of the setting to find alien tech, but it is an option; and who doesn't like exploring a perfectly static alien-death-cube in orbit around a comet?