Click on your avatar in top right corner, go the App store and under Productivity you can toggle on the Timesheets field for your cards. You can now add activities to a card's timesheet.
You can also edit all timesheet entries from this same view.
Timesheet totals will appear on the card,
and in the card.
Timesheet summaries will appear in column and board headers as well as in lane names if enabled.
If on the Enterprise plan, you also have access to powerful timesheet reports.