How to extend the day past midnight to complete the activities and close the rings
If you need to extend your day past midnight to close the activity rings set temporarily the timezone to a timezone farther west. Change the timezone before midnight. Now you can use the additional hours the timezone is shifted relative to your usual timezone to complete the missing activities.
I tested this again during a long distance flight, where the timezone at the destination is shifted by 8 hours relative to my home timezone.
As you can see, watchOS 2.1 will happily give credit for activities for more than 24 hours if you shift the timezone.
At 5pm the watch already showed 25 hours of "Stands", more than one day.
And with the 8hrs offset to my regular timezone I could accumulate 30 stand hours within one day's work:
If you want to extend your day to complete activities this way, don't forget to set the timezone back before you go to sleep, or you might forget the next day that the watch is a few hours off. I would wait to set the timezone back until after receiving the message that all tasks have been completed for the day and set the timezone to the correct time after the next hour.
MacBook Pro (Retina, 15-inch, Mid 2015), OS X El Capitan (10.11.4), 2,8 GHz Intel Core i7, 1TB SSD