I've found that when I'm down, in a rut, or tired, doing simple acts of good is one way to help me feel better. I occasionally make donations to GiveWell (on top of my monthly donation) or set up blood donation appointments (effective insofar as it comes out of the time I would spend reading at home, but instead read while giving blood). Are there are activities like these that are low effort but have high emotional payoff for me and reasonably high altruistic payoff for others?
Going for a walk in an area with some litter, carrying a small trash bag with you and wearing gloves. Cleaning up litter is an extremely visual/sensory way of making an environment better, and while the payoff isn't too high (other people have nicer walks), the effort + sense of accomplishment make it a big mood-lifter for me.
I also second Denise's thoughts on calling elderly relatives, and would extend that to "getting in touch with someone who seems to need a friend" -- whether that's a parent, an out-of-touch sibling, or an old friend you haven't seen in a while and suspect might want some company.