Do you all live near each other, or are you remote?
If you live near enough each other, take a whole day off from your project to do something fun, like go to an amusement park.
If you live near enough each other, take a whole day off from your project to do something fun, like go to an amusement park.
My point is that if you take some time off to do something fun together, then on the days when you are working you'll be more productive.