your formula is only telling it to sum all of the records (then SUM(field))
IMO I would use a crosstab. It is easier but you could also use Runnning totals, 7 formula fields or vairable formuals.
The crosstab would be
column data use the datefield and set the grouping to 'for each day'
summarized field use table.activity set to a SUM.
play with the formating to make it what you want.
You can alter the day headers to use the weekdayname instead of the date.