I've still got a 4x4 piece of plywood leaning up against the wall in my apartment. The original idea was to build a small table layout, but now I've gotten to thinking about L- or delta-shaped layouts, made to fit in the corner of a room. Here's one idea. it's a simple twice-around, up and over, with a passing loop and a few spurs. Curves are 12" and grades are 3-4% (depending on your desired clearances) except for the upper spur, which is 6%. If you're modeling the US, this spur goes to a logging camp. If you're modeling the Continent, it goes to a ski area and scenic overlook. If you're modeling Japan, it goes to a temple that has been continuously rebuilt for the last 1200 years.
I think it's a great little layout that will provide some nice times. Short-line w/ a handful of cars...little bit of switching...I like the logging idea for the upper line but that's cause I'm from the NW. Could be a small mine or a granite quarry. Might be interesting to make part of it as a very steep incline, with bevelled cars being pulled up/let down via cable. Transfer at the base just inside the switch to regular flat cars.