Modsy is a service that can do it, but it doesn't let you do it.

How about plain ol' graph paper, a ruler, and a pencil? If each box represents 1 foot, sketch out the wall dimensions and door/window locations. Then put tracing paper on top to put in the furniture you've selected. One sheet of tracing paper for each furniture layout option.

That's how it was done before CAD; it's an easy way to check the dimensions of the furniture to make sure it all fits.

I'd like to figure out how to get a 3D color rendering. I tried the online sites, but found them way too cumbersome and difficult. A simpler solution would be great.

3D you need a plan layout and in order to do a 3D rendering of the room or whatever you like to render here is an example

Room layout

Rendering