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Relatively simple:

Catwalks that can be built over top of normal buildings that lessen the movement penalty of the squares they occupy. This would solve the problem of building intricate pathways around batteries and hydroponic farms and would save space that could be used better for more buildings.

Disco Ball: Induces a euphoric state on your space cowboys.
Barbed Wire: Cuts the clothes of those who cross them. Would provide a little cover.

Shooting range: Hones the skills of those with guns when used by them.
Colored lights: Purely aesthetic, or could influence colonists' mood. Heck, they could even change color.

I'll think of something else later.