Pre-built Mars bases.

[Alshain]

Looks like an interesting idea, basically use 3D printing to create Mars bases before any people actually arrive.

[Zuni]

But where do you get the printing goo?

I guess it would still be a lot of work for the humans, making it all airtight and fitting doors. But overall this is a cool concept.

[Tommygun]

The glue or goo would be sent up from Earth, but you are mixing it with local soil so you don't need as much building material from Earth.

It's an interesting idea, but personally I think making bricks from the soil and firing them in a solar oven is a better choice for the Moon at least.

Mars is a lot farther from the Sun. You would still need to glue the bricks together, but that would take less glue than the other method.