Here's a fun little command from the AutoCAD days...
_aeclineworkshrinkwrap if you prefer. We use AutoCAD ADT 2008 at our office, but I think this command goes back a long way (and probably will go forward forever if I know Autodesk). What it does, as you might infer from the name of the command, is create a closed Polyline around the things you have selected. You may not have an immediate need for it, but I can assure you, once you know it's possible you will find an application.
I don't actually know where the AEC commands are documented (if you know, please post it in the comments), but I have found scant references to it in the forums and in the Civil 3D online help. There are other gems hidden in there too such as
AECTOACAD for those times when you need some help cleaning up a dwg.
Here's a bonus tip - if you can't quite remember the name of a command in AutoCAD, start typing it on the command line and then hit TAB. You will be scrolled through all possible commands that begin with what you typed... a good way to jog your memory or just see what's available. Also very telling about the origins of the software.