I keep a diary by using journal.el -- it simply creates a new .org file everytime I call M-x journal with the current Date timestamp (date_day_of_week.org). Works fine. I usually create a * tags headline and tag it with relevant tags. The downside is that, in order to have all the agenda power, I need to manually add it to the list. Not sure if there is a way to include a whole directory automatically somehow?
Some of you might say to keep the journal in one big file. I just don't like it. Separate files make things more maneageble for me.