There are websites whose entire reason for existing is auditing charities. You can always go for something like that.
Donating your time can be very inefficient though. Imagine you are in software, and you'd be donating time to move boxes around in a food pantry. They could hire someone at minimum wage to do this,(which is actually good for the economy) or, you could be doing some consulting, and donate that. It's also the reason you don't buy groceries for the pantry, but hand them cash instead: They will get them far cheaper than what you'd pay.
The way I donate time is to work more hours, and specifically dedicate some contracts to be directly donated to charity. Sending a check doesn't get the same public recognition as donating time at minimum wage, but it can be far more efficient, and IMO, donations are about helping a cause, not improve your social status doing it.