A will's main purpose is to legally state what you'd like to leave behind to beneficiaries. There are things not legally enforceable in a will, such as:
Funeral arrangements
Providing care for beneficiaries who have special needs - this will need its own separate document, called a special needs trust
Setting certain conditions for inheritance - you may set conditions for an inheritance to your beneficiaries as long as these conditions are not based on religion or marital status
Providing funds to any illegal activities