Ok, this has got to be one of the easiest Mitzvahs:
There is a Torah prohibition against owning Chametz (product made from leavened or fermented grain). Because we can't always get rid of all Chametz, there is a simple solution which involves selling our Chametz to someone who isn't Jewish for the duration of Passover.
It is free (donation optional), it is important and it takes only a minute: