So your layout has progressed beyond the plywood plains to include some basic scenery and a few structures in the city. But there aren't any tenants in your buildings yet, right? If you don't want to model interiors and want to have windows that you can see into, you're going to have to find some way to explain the empty spaces within the buildings. While you could put rental or sale signs outside the buildings, but it isn't entirely prototypical to have every building with such a sign in front of it. Try this strategy I saw on an HO scale layout recently: place a delivery truck outside the front door of one of the buildings and have some delivery men moving furniture into the building. It not only provides a reason for empty rooms, but also gives visitors to your layout something interesting to look at.