For the activity log to have any factual basis, you need to be yourself at work. Don't try to hurry your tasks (effectively sacrificing quality) and do not feel under pressure because you want encouraging results. Nobody is going to see the log at the end of the day but you, so there is no need to impress anyone. Also, the purpose of the log is to get a snapshot of a regular day's work, not an extraordinary one's.
Important work tasks are not the only activity you should log. If, for example, you spend time searching for a blank sheet of paper or going through your emails, write it down. It may only be for seconds, but your brain needs to focus back to work and if these distractions happen quite often, it can cost you several minutes of your workday. Note down all recurring trivial tasks so that the next day you can bring a pack of blank papers and set up an automatic email notifier, for example.
When at work, it is not always easy to remember that you are also recording yourself, so you may forget to write down when you started a task and when you finished it, or even leave successive hours without an activity. For this purpose, keep the log in front of you, next to your keyboard or anywhere you can see it. Avoid keeping it on your briefcase or purse, as quick transition from one task to the other won't let your brain remember you have a log to keep.
Include comments on how you felt after completing each task. Write briefly about how easily you did the task and how focused you were. You can also do this by rating your willingness to work on a 1 (dreadful) to 5 (pleasant) scale. This way, you can later evaluate your most productive hours in the office and place the most important tasks at this point in the day.