That is a probably an excellent idea for by keeping a diary you may be able to keep track of your activities for the day and thus be persuaded to lead an examined life which conforms to the noble standards of Islam. It is also a good way of putting to paper all of your life experiences for the benefit of yourself and others. We read in the sources that when of the literary giants in Islam had passed away one of his friends saw him in a dream. The latter asked him about the advice he would offer him not that he is dead and is enjoying the fruits of his own life's work, the former said, "Don't write down anything with your hands except that which you would like to see in your register of deeds on the day of reckoning!"
Umar, the second Caliph, used to say, "Examine yourselves before you are examined before you face the examination by Allah." Thus, we are told that he was in the habit of writing down his activities for the day, and he would review them before retiring to bed to save himself of the agony of the rigorous examination on Day of Reckoning.
So, go ahead and keep a journal of your daily activities, and let it serve as an inspiration for you to make every day better than the previous one.