There is a related practice where you can switch your current account. At least in the UK, there are several banks that will pay you from £100 to £150 for switching your current account to them. Like the above practice of churning, this will have a small negative impact on your credit score in the short term, but is otherwise fine. The bank will do all the switching for you, and transfer all your direct debits, standing orders, etc, from the old account. Once you have received the switching bonus (normally within a month of the switch), you can then always switch back. Naturally, you should choose a no-fee current account to switch to (all of these offers allow that). Note that some of these switching bonuses require you to have at least two Direct Debits and pay in a certain sum of money per month, so read the fine print.
The following offers are currently available in the UK (not necessarily an exhaustive list):
What you do with the money is of course up to you, but this is compatible with effective altruism. I have taken advantage of all of these offers at one time or another, and heartily recommend it.
EDITED TO ADD: Many of these bonuses cannot be claimed if you have already received the bonus too recently. So it's not like you can make £600 a month doing this!
For those of you unfamiliar with Churning, it's the practice of signing up for a rewards credit card, spending enough with your everyday purchases to get the (usually significant) reward and then cancelling it. Many of these cards are cards with annual fees (which is commonly waived and/or the one-time reward will pay for). For a nominal amount of work, you can churn cards for significant bonuses.
Ordinarily I wouldn't come close to spending enough money to qualify for many of these rewards, but I recently made the Giving What You Can pledge. I now have a steady stream of predictable expenses, and conveniently, GiveWell allows donations via most any credit card. I've started using new rewards cards to pay for these expenses each time, resulting in free flights (this is how I'm paying to fly to NYC this summer), Amazon gift cards, or sometimes just straight cash.
Since the first of the year (total expenses $4000, including some personal expenses) I've churned $700 worth of bonuses (from a Delta Amazon Express Gold and a Capital One Venture Card). This money can be redonated, saved, spent, or whatever.
Disclaimers:
1. I hope it goes without saying that you should pay off your balance in full each month, just like you should with any other card.
2. This has some negative impact on your credit, in the short term.
3. It should be noted that credit card companies make at least some money (I think 3%) off of your transactions, so if you're trying to hit a target of X% to charity, you would need to donate X/0.97, or 10.31% for 10% to account for that 3%. The reward should more than cover it.
4. Read more about this, including the pros and cons, from multiple sources before you try it. It's not something that should be done lightly, but does synergize very nicely with charity donations.