I've been a volunteer treasurer for several medium sized non-profits.
You should have a think about what sort of charity resonates with you. Like education support, homelessness, legal support, drug rehabilitation, et cetera.
Then call a few in your area and tell them you're considering a bequest, and would like to understand some more about their organisation and how the money is used. 1
DepYou could attempt the non-selfish kind and just donate it all to an independent health/rescue org like Red Cross/Red Crescent.
You could also go the kinda-selfish route like Alfred Nobel, known in his time as the merchant of death. Make an elaborate award&grant giving scheme for exceptional contributions to society in a variety of fields; boosting said contributions for many years. Would only recommend this route if you've got more than enough coin to spare, as the overhead of ensuring ethical operation is significant.
If you've got a house, you could transfer ownership to a trust/foundation/housing coop, to make it available for living at below market price.
I'd donate to various free software & open hardware projects important to societal improvement; like Mozilla, certain fediverse projects, PostmarketOS, Fairphone, etc.
Also anarchist orgs.
Find an organization with a long history of successfully bringing meaningful change to society and that has stayed true to its values. In my opinion, the Electronic Frontier Foundation comes to mind.
Or find something local to your community, directly giving to actual families in need near you
One quirk of the laws in my country is that you can't have arbitrary wills. Your family will inherit your possessions, and the order in which their claims have priority is also established. A will helps that process go more smoothly, that's all.
The only guaranteed way to make sure your estate goes where you want it, is to enact the transfer when you're still alive. Which has the added benefit of allowing you to see the impact it makes, so maybe this is a good idea in other places too.
That’s also what general happens in the US if you die without a will. It’s called intestate succession. If your spouse survives you, they will generally get everything held as community property. If you had property prior to the marriage, it might be divided among your children, if any. Of there are no immediate family (spouse and/or children), it can then be divided among any relatives you do happen to have.
If you have any valuable assets that you want to bequeath to someone in particular, then it’s important to have a will. If you want a bigger chunk of money to go to a relative you know is struggling financially or could otherwise use it (kids heading to college, buying a house, whatever), then that’s another reason to have a will. Of instead of a family member, you want the money to go to a charity, then that’s yet another reason.
Donating to just about any charity is a noble cause! I'd also suggest establishing scholarships with educational organizations you like and, depending on the size of your estate, consider endowments that ensure continued support to those organizations.
Enough of the suggestions appear to be good; I'm going to say one that I don't think has been mentioned yet - - St. Jude. They help kids with cancer & their families, so they can get treatment without paying anything out of pocket. And I mean ANYTHING.
Families with young kids tend to be on the poor side. Cancer hits them like a ton of bricks; they can't afford this expense. St. Jude pays for transport, puts the family up while they treat the children. So much money is given to the kids & their families.
It's not all altruism & butterflies, though! It's also just...fucking smart & helping to refine modern medicine. Millions of USD goes into R&D, working to treat & cure cancer, in kids. Hopefully creating tech, methods that benefit us ALL. There is no downside to donating to St. Jude.