Nissen huts were used for a variety of purposes, most commonly barrack accomodation, stores, base hospitals, general office space, pretty much anything you can think of.
You can't go far in the UK without encountering Nissen huts, either on airbases, current and desserted, farms and often aften having been moved in urban settings.
On the subject of bycicles model railway suppliers make a large range of bikes in plastic and white metal, would certainly save the need to scratchbuild.
And if you've got bikes you need bycicle sheds, also available from model railway suppliers.
Don't forget trenches and air raid shelters on a base, especially in Europe, as well as pill boxes around the perimeter. Where I live I am surrounded by ex RAF bases, all of which are ringed with pill boxes and other associated bunkers.