I suggest a trackplan that provides a lot of switching. The prototype tries to avoid switching when possible because it is a very labor-intensive ($$$) process but on a model pike it provides variety and visual/mental stimulation. The Plywood Empire route does provide continuous operation with one simple loop around the perimeter of the room and I do indulge in some railfanning 
(the D&RGW Royal Gorge rolls through daily and the daily freight makes a couple circuits on both the incoming and outgoing run) but the bulk of operation is the purposeful set-out of deliveries and pulling of empties/loads with subsequent makeup of a train. Interchange is a return-loop in the next room. I too love construction and tinkering but I find operation satisfying as well. About 30 cars are in-play as is one engine and one crummy. ON EDIT: plus the Royal Gorge, made up of an F3, Grill/Lounge and Coach/Dome. A PW Lionel Gantry Crane provides operation of a different kind: the loading of actual scrap steel into Standard O gondolas. Once at interchange the old 0-5-0 handles the return of scrap to the scrapyard.
I find Frank Ellison's book a must wrt trackplans and prototype-like operation:
From time-to-time one comes up on eBay in the 15$-25$ range.
Lew
