- GUIEVENT_MINIMAP_MAPPIN_CLICK: The waypoint the map note is attached to.
- GUIEVENT_CHARACTERSHEET_*_SELECT: The owner of the character sheet.
- GUIEVENT_PLAYERLIST_PLAYER_CLICK: The player clicked on.
- GUIEVENT_PARTYBAR_PORTRAIT_CLICK: The creature clicked on.
- GUIEVENT_DISABLED_PANEL_ATTEMPT_OPEN: For GUI_PANEL_CHARACTERSHEET, the owner of the character sheet.
Anything not listed above has no additional integer properties, whereupon this will return OBJECT_INVALID by default.
The GUIEVENT_CHARACTERSHEET_*_SELECT matters since you can open the character sheet of familiars, animal companions, summons and henchmen.
While you'd think GUIEVENT_PARTYBAR_PORTRAIT_CLICK could never return the player themselves, it can do if they first possess their familiar, which puts their PC character in the party bar.
This function was added in 1.85.8193.30 of NWN:EE.
See OnPlayerGuiEvent for context and an example.