Hey, I was actually looking into this a few months ago and the option I landed on was to simply set up an account at a given exchange in the charities name and use them to convert to fiat. So, you would provide your visitors with the address which the exchange generates for you.

If you're in the States (and probably elsewhere) then make sure to get something from visitors which explicitly state what the fiat value is of their crypto donation. This will help with any tax filings you need to make.

https://www.bitgo.com/ may be an alternative but I didn't need their more advanced features.