Well there have been more then a few cases of "blue ice" falling from the sky because of leaks.

Not counting fuel, there will probably be a slight difference from the amount of moisture we give off in breathing, assuming that's vented out of the plane and not just absorbed in the fabrics.

I also believe that the air introduced into the plane is humidified a bit, so there would be some water vapor added to it, as that circulates and is exchanged some moisture may escape, causing the weight to drop a bit.

There may be some fluids like oil and grease that also burn off a bit, friction and that, again probably trivial amounts.

But besides that, I would assume that everything is a closed system.

In reality though on planes that have in flight internet, all those extra electrons from people downloading videos may actually add to the weight when it lands.

Kidding about that part.