Go directly to the (charity,do good, religious, or whatever) and tell them you want them to have what's left. They will either do it themselves or have their own rep do it. Avoid going through a "phone book" lawyer and getting ripped off. Avoid probate , and here the major recipient will most certainly advise and help. The charities will be able to help you create an estate plan that gets them the money while avoiding probate. Depending on the charity, they may have volunteer attorneys that will assist you for free.
You can ask...worst thing they can say is "no."