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If you look up Expedition To The Barrier Peaks it’s a dnd module from the 80s where the party investigates a crashed spaceship and deals with robots and aliens, it’s still pretty good I’d say

You might be interested in #rifts setting

https://therpjournal.wordpress.com/2025/03/26/rifts-an-underrated-gem-in-the-world-of-tabletop-rpgs/

The game takes place in a post-apocalyptic Earth where dimensional rifts have torn open, bringing magic, monsters, and alien technology to the planet. This fusion of genres allows players to encounter everything from high-tech cyborgs and vampires to dragons and ancient deities. Rifts offers an unprecedented amount of flexibility for creating characters and stories. You can be anything from a power-armored soldier to a mystic who commands the forces of magic, or even a dimensional traveler with access to alien technologies.

D&D actually has its own unique space aliens; I’ve always just used those because everything about space in D&D is gnarly. There are a lot to choose from. Some entirely unique to D&D, others inspired by popular media like the Cthulhu mythos.

For mechs you have all sorts of golems, clockwork/steamwork machines and races like the Warforged.

Honestly, I’m half thinking of running mines of phandelver, but replacing the black spider with a Warhammer 40k crossover: The Patriarch.
Genestealer Cults infect the local population, preparing for the main invading Tyranid forces.

For your average d&d party, stuff like getting zapped to another plane of existence, potentially including literal hell or a plane made up entirely of gears, isn’t an entirely unexpected circumstance. I played in a game one where due to some crazy shit happening to us in the feywild we ended up in sort of the backrooms of the D&D multiverse with an impossibly long tapestry hung on the wall that was a physical manifestation of the weave.

Between those sorts of hijinks and flying around on spelljammers through interplanar space with crystal spheres and all of that mumbo-jumbo, aliens, space travel, and alien technology almost look tame in comparison.

I feel like your best bet to really confuse the characters is probably to have aliens simulating a normal d&d environment for them- maybe the room they wake up in on the ship looks very much like a stereotypical d&d tavern, but little details are wrong. Maybe there’s a bard strumming away on his lute but his fingers don’t quite match up with the notes he’s playing, and the bartender asks if the party would like a “brewed and fermented barley beverage” instead of an “ale”

And the clothing the rest of the people there is all strangely generic and maybe a little dated and things are just generally not adding up, like why is there a red wizard from thay happily playing cards with a waterdavian city guard, and why is that guard drinking in his uniform? and are we even in waterdeep right now? because the elf at the table over there is reading a baldurian newspaper, and also holding it upside down. And is it just me, or are there a whole lot of twins and triplets here?

But things like detect magic aren’t turning up anything fishy because it’s not magic, it’s science.

And maybe when you try to tap someone on the shoulder to ask what the hell is going on, you phase right through them because they’re all holograms. A simulation orchestrated by the alien abductors to help the party feel more comfortable and so that they can study them in something close to their natural environment.

You might try the novel Eifelheim for inspiration. In 1348 an alien ship crashes in the Black Forest. It’s wild exploring how the Jesuit priest, peasants and villagers react. The author throws a lot of medieval trivia around and it’s fascinating.

If you’re open to branching into other systems, the folks at !scifi@ttrpg.network can help you pick an easy to learn and vibe appropriate system for your adventure.