Make the Billable selection a check box on the time item cell itself
Currently for each cell in the timesheet you have to select the cell, click the Billable textbox, select Yes/No, then click Save. This is a four-click task for EACH cell.
This could be reduced to a one-click task for each cell by making the billable selection a checkbox in the cell itself. This would make it easier and quicker to set the billable on each item, as well as allowing the user to verify, at a glance, that the sheet is correct before submitting.