Here's a really simple technique for managing short-term to-do lists in Emacs with outline-mode.

I start the day by creating a new to-do flie in outline-mode. I use different outline levels to keep tasks organized by area. Since I have font-lock-mode enabled by default, all of my tasks end up in various colors. When I complete a task, I simply prepend a space to that line in the file. As Emacs no longer considers the line an outline heading, it loses its highlighting, thus marking the task done.

For instance, here's what today's to-do list looked like at some point this afternoon:

There are certainly more featureful ways to manage your to-do items, but this is an exceptionally simple method for handling more ad-hoc tasks that come up during the day's Emacsing.