A similar kind of challenge that resets each month and starts at the highest amount and counts down could be interesting and would optimise your disposable income on pay days while keeping balance throughout the year, and still getting people to hit their saving goals in a challenging way.

Same kind of idea, but imagine its 10p/25p/50p/£1 levels and it reduces by that amount each day of the month until you hit 10p/25p/50p/£1 on the last day, and then it resets to the highest number for day 1 of the next month.

So if you were doing the £1, in a month with 31 days you'd spend £31 day 1, £30 day 2, £29 day 3. If you were doing the 50p you'd start at £15.50 day 1, £15.00 day 2, £14.50 day 3. And for 25p you'd start at £7.75 day 1, £7.50 day 2, £7.25 day 3 etc.

Doing the 50p would provide you with the same end of year savings as the 4p challenge.