To better budget your limited amount of total hobby time, maximize your railroading time by minimizing your modeling time.

Project Books

Model Railroading (*.pdf book)
Structure Projects for your Model Railroad (*.pdf book)
Illustrated Layout Projects (*.pdf booklet)

Papercraft

Papercraft is the oldest form of railroad model structure crafting. Paper is cheaper than wood or brass, it does not require painting like plastic, and thanks to modern color printers it can display a level of illustration detail once only available on lithographed tin. Port scenes are especially suitable for Papercraft, as you can inexpensively build full O-scale merchant ships and waterfront facilities (see page 61 in the book "Waterfront Terminals and Operations"). Regular paper will absorb humidity though, so to prevent book mold or sagging, use one of the modern heavier-weight treated papers (or better yet, cut and paste the paper onto styrofoam).

Building paper and cardstock models (*.pdf)
Modeling with Cardboard (*.pdf)
PaperCraft building construction ideas (*.mp4) - {take extra care to keep seams sealed flush}

Simple Projects

There are a few O-scale pre-assembled plastic structures available, but they typically will be undersized in regards to the footprint required by the business indicated. Although these pre-assembled structures are the quick solution to decorating your layout, the better solution is usually to custom construct (including when building from a kit) your own structures, so they will fit your layout vision exactly. Unless you are building a diorama for a photo shoot, handling durability should always trump fine detail fragility.

Build a Sanding Tower (*.pdf) - {a simple, but useful, first project}
Modeling a modern tankcar transload (*.pdf)
How to Build a Simple Rail Barge (*.pdf)
Section Houses (*.pdf)
Box Car Out Buildings (*.pdf)
Model a Freight Shelter (*.pdf)
Concrete Bag Retaining Walls (*.pdf)
Model a cattle guard (*.pdf)
Tree Stumps (*.pdf)
Make Easy Road Signs (*.pdf) - {sign poles are often slightly askew, and road-edge signs often have impact damage}
Save the signs (*.pdf)
Modeling a railroad radio base station (*.pdf)
Scratchbuild a Taxicab Office (*.pdf)