A good grain facility (one of my favorite) can be modeled. They are not overly complex, provide a lot of traffic in the fall, and have a lot fo fun structures. Your height keeps you from modeling a mega grain facility. They also have a lot of options for a receiving industry for your grain.
Depending on the area and era you are modeling, livestock might be a good option as well. Lumber and mining are easy in a rural setting and you will find no shortage of help with either of those options. All three of these could have a raw material area, i.e. cattle, logs or ore, and the area they are processed at, meating packing or auction facility, lumber mill or a smelting operation. Era will dictate a lot in this area.
The model I am doing is a small grain, lumber,hardware, coal and heating oil facility. The main structure has the first four industries side by side in one large building. It is a rural store that serves the surrounding area.
Since this is something that has to hang out under your bed, you could also try the suggestions on this thread:
http://www.the-gauge.com/thread14228-mini-layouts.html
It was a discussion of using a mini, which could hang out under your bead, as a hub (since you have all the area under your bed you could enlarge the idea of the "mini" into "mid sized"). You can then make some other layouts that connect to the hub. These can be stored on a shelf or elsewhere and can be run as stand alone model railroads. It also has the benefit of having finished areas to run trains on quickly for when you want to run trains instead of build.
Hope that gives you some ideas!