There is not much that cannot be covered by your will, which makes it much easier to list what cannot be included, over what can be. The three basic things that you cannot include in a will are as follows:
Stipulations requiring marriage
Stipulations requiring divorce
Stipulations requiring a change in religion
You cannot legally include any of these three stipulations in your will as requirements for someone to receive their inheritance. If any of these stipulations are included in your will they cannot be upheld in court and therefore the person these stipulations have been placed on will still receive any inheritance left to them regardless of the stipulations.
Some important items that can be included in your will include custody of any minor children or adult dependents in your household, the creation of living trusts from your estate, and even your wishes for your funeral, or lack thereof if you do not wish to have a funeral.