We had P-day on Mondays. We actually called it P-3 hours because we still had class in the afternoon. In the morning after breakfast we would go to the temple, which was always really nice.
After class that afternoon we had dinner and then free time for the rest of the night to do laundry, write letters, etc. There wasn't really a whole lot to do since we couldn't really leave the MTC, so a lot of people would just hang out in the laundry room or their dorm rooms.
One P-day we found The Pillow Room. It was just down the hall from our dorm room and we think it was usually locked, but this time it wasn't. This room was full of pillows; we couldn't even reach the floor when we jumped into the pile. I think we played in there for a good 20 minutes. It was a great stress reliever.