Schoolwork--I just had a killer exam yesterday, and have been way too stressed/tired/underfed (happens when I'm stressed and tired) to do anything. Usually it's a matter of forcing myself to get out if I'm feeling okay about my progress. If not, I just have to weather the storm and go back out when I'm better nourished and rested.

The weather's the other one. I'm not heat tolerant, and I have an abdominal muscle injury from November that likes to flare up again if I push too hard, including riding when it's windy. I'm okay with getting caught in a sudden shower, but if it looks like it's going to rain, I won't go out. (Of course, once I've decided against going out, it clears up.)