Just came up with a nice idea for a very good first experience:

Put actual money in the Jar. When the year ends, make the donation through Check or Online.

Match the donation you just made with a donation for yourself, an yearly gift that your previous altruist selves gave you to spend in happiness increasing activities. That is, right after finishing your donation, open the Jar, put the money in your pocket, and start thinking about how to invest it in happiness.