For each resource, on each date in the calendar, you can click the "plus" symbol at the bottom left of the cell to add a new activity (this becomes visible when you roll your mouse over the corner of the date—as with availability, it too will only appear for current and future dates, not past dates).
The "Add Activity" option will open the "Create Activity" modal:
Activity type, start and end date/time, and address are all required fields.
The "Billable" checkbox will determine whether the activity is included in the scheduled hours for a resource.
Checking this will include the activity in the resource's scheduled hours.
The "All Day" function will make the record a 24-hour duration for the date selected.
The "Repeat" function will enable a recurring pattern of activity to be created on a weekly or fortnightly pattern. This can be repeated for a number of periods, and any global holidays can be skipped.
Like with availability, right-click on an existing activity record to edit it. Selecting "Edit Activity" from the drop-down menu will open the "Edit Activity" modal. If the activity forms part of a recurring schedule, the "Edit Recurring?" checkbox will be displayed—when checked, you have the following options:
Same Weekday (<Day>): Edit all records on the same day of the schedule.
Following Events: Edit all future records of the schedule, from your selected activity onwards.
All Events: Edit all records of the schedule, even those that occur prior to your selected activity.
The "Delete Activity" button is also available on the "Edit Activity" modal. If the activity forms part of a recurring schedule, the "Delete Recurring?" checkbox appears on the delete confirmation pop-up.