First of all it highly depends on the user. I for example have an activity "Sleeping" which I activate on going to bed, and it is normally followed by guess what: "Breakfast". And I somehow believe, that after all the bigger the knowledge (or amount of data) the diary has, the bigger the  benefit. But I also have to admit, that I have an activity "Relaxing" in my case, which is also a box for everything I intentionally do not want to track.

Statistics are so far not really there, and we have to support there such "\<No Activity>"-activities, e.g. by allowing to filter them. But also I think the first statistical view that I will implement is on the activities itself, so if you are not interested in how long you did "nothing" just don't look at it :-)

And on the other side there is a semantics of having "\<no activity>" selected, you might remember it from the very first start of the app. And each entry in the diary has a start and end datetime. After an end and before the start of the next entry there is "\<no activity>". The current UI doesn't allow to select this, as the current activity is always "terminated" by starting the next, but as soon as history-modification is implemented there will be a way to reach that anyhow. So maybe we should add a way to directly just "terminate" the current activity without starting a new one?