org-agenda-log-mode-items

org-agenda-log-mode-items is a variable defined in `org-agenda.el'.
Its value is

(closed clock)

Documentation:
List of items that should be shown in agenda log mode.
This list may contain the following symbols:

closed Show entries that have been closed on that day.
clock Show entries that have received clocked time on that day.
state Show all logged state changes.
Note that instead of changing this variable, you can also press `C-u l' in
the agenda to display all available LOG items temporarily.

You can customize this variable.