One of the simplest and most powerful way to make your 3d game more interesting is to add realistic textures to simple objects in your game. In this tutorial I show how to add image textures to the faces of a cube. This allows you to easily create a tiled floor, or a grassy plain, or a brick wall. It is also used to create skyboxes (though in that case the textures are put onto the inside faces of the cube rather than the outside faces!)
On this page we show how to create a tiled floor and a building. The screenshot below shows several copies of the tiled floor and the building from the avatar's view with a skybox in the background.
Watch this video and then create your own textured objects for your game. You probably want to create a library of useful objects (e.g. different kinds of floors, walls, etc.). This video requires Blender 2.5 and a web browser so you can download textures (or you can use your own image files).
Here is another video tutorial which shows how to use UV texturing to create an apartment building
This also covers adding the object to a group, and uploading the zipped file to a google site page.
Here is a screen shot of the building we create in the video above: