An organized room.
Organization is a vastly under-appreciated state of being--until you don't have it. I've recently discovered that organization is one of my very most favorite things.
Today, my room is finally organized (okay there are a couple of little things on the dresser that you can't see that still needs some attention, but it's mostly good). I've discovered the clutter in my house is directly proportional to the clutter in my brain. The more items that have been put in their proper place, the more space I have in my brain.
Organization saves time--no more searching for things. Organization saves energy--no more searching for things. Organization improves mood--no more frustration while searching for things. (I bet you're beginning to see a trend here, eh?) Organization feels successful. It will be a long time before I take it for granted again.