I don't play CSS, but I'll still throw an idea out there: if you must have double points, pick a random day of the week after the week is over to double. In other words, after the last playing of the week (usually Friday, but not necessarily), roll a die or go to random.org or whatever and pick a random number from 1-N, where N is how many games you played that week. (For example, if you played all 5 days, N = 5, and you'd pick a random number from 1-5.) That day's playing is then retroactively doubled. That keeps one doubled playing per week as well as encourages people to play more, because they don't know if that day's playing will be doubled or not.