Here are some things that I find to make nice diversions that don't take many parts or much time: - Figbarfing, as AK mentions. - Brick-built animal. - Blackjack: Build something with no more than 21 parts (but as close to 21 as you can get). - Do a study. Graphical artists do studies of little details, like a mouth, and hand, the wing of a bird, etc. LEGO builders can do studies, too. Try a new frame for a door. A new outline for a window. A new motif to go under the machicolations of your next castle. A new type of crenellation. Maybe some running water. A new type of bush. A new type of tree. Your goal is to make something that you like well enough to incorporate into your next full-sized build, but not to turn it into a build of its own. You're just experimenting with techniques to see what works, and what ideas it gives you for your next build.
I am giving Josh a break this year and coordinating castle at BrickCon. Official "categories" were submitted today for those that care about such things. I put us in for Best Fortification, Best Castle Scene and Best Old School.
If anyone is interested in helping with trophy building, let me know.