Charity when time isn’t convertible to money?

Having just re-read “Money: The Unit of Caring”, I noticed that the general methods proposed therein make some assumptions which don’t seem to apply to me, and I’m trying to figure out how the conclusions therein change therefrom.

Avoiding certain personal details, I’m on a fixed income; I get a monthly deposit in my bank account every month. I don’t expect this to change in the foreseeable future; and at least in the general sense of ‘job’, it’s unlikely I’ll be able to acquire one. In sum—I don’t have any easy way to convert my time into additional money.

However, I still want to get the occasional warm fuzzy from causing the most possible good from what I can do—even if that involves my volunteering to spend some hours of my life doing things that would be inefficient for someone else. For example, donating blood, or taking an overnight shift keeping an eye on things at the local ‘out of the cold’ program; and using givewell.org as a guide for what money I am able to funnel into direct donating.

So—does anyone have any advice? (Or questions that would help better advice be given?)