If you care about non-human animal welfare The Humane League Fast Action Network provides quick and easy actions to help their corporate campaigns. In their words:
We simply send you easy, online actions, such as signing a petition, posting on social media, or emailing decision makers. In a matter of minutes you can drive our campaigns forward, all from the comforts and convenience of your home, office, or life on the go.
Something I enjoy doing and that really lifts my mood is editing Wikipedia. One reason why I enjoy doing this is the feeling of contributing to a community and "adding" to this huge collection of knowledge. It is also plausibly at least somewhat impactful. Wikipedia is often one of the first sources people read on a topic if they want to know more about it. There are a Lott of low hanging fruits, especially, if you are a non-english speaker since the articles relevant to EA's are often quite bad in the non-english Wikis. Here and here are overviews of articles that could be improved or created. For the warm glow of adding to humanities knowledge you can also add to non-EA related articles (e.g. your favourite not-super-famous-Band, a topic related to a term paper you recently wrote etc.). This sometimes feels easier since you don't have to think so much about framing the topic perfectly.
I've been a more avid wikipedia editor in the past, thanks for reminding me to get back to it!
This is a bit of a frame challenge, but I think it's OK to feed stray cats. Most people are built to empathize with people around us, not the total sum of global utility, so it's hard to beat the emotional high of a simple random act of kindness. (Conversely, for most people, the vast majority of good you can do comes from your career choice, and it's hard to approach this with small-scale actions.) So my advice is to pick someone close to you, do something nice for them and not worry about the magnitude of the altruistic payoff. You could also reflect on the positive long-term impact of some action (mentally follow the chain all the way from "finish project" -> "gain career capital" -> "get hired by <EA org>" -> "be able to work on <cause area>" -> reduce suffering) and use that to motivate yourself, but that only works for some people.
This is a classic idea in EA circles going back to 2009, and it absolutely still applies.
Related to doing something nice for people close to you, I was reminded of Neel Nanda's post where he encourages the idea that helping friends to become more effective might be really useful. Doing this probably gives warm fuzzies and doesn't need to take longer than a call or taking a walk together.
In the spirit of Aaron Gertler's expansion on calling elderly relatives, we can extend "feeding stray cats" to spending time with animals. This can be as small as giving some extra attention to local animals--in my case, I like to hang out with the cows and sheep at my university who are destined to become meat--or as significant as volunteering at a farm sanctuary.
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.
Crisis text line is a non-profit organization devoted to the idea of providing someone to talk to when you really need one. Typically as a crisis counselor you will log on and join the "queue" of people waiting to talk to someone who needs it. When people feel overwhelmed (In crisis) they'll text in, those texts are sent directly to the web browser of the person next in line, and pop up as a chat box.
I can't resist mentioning that Mahayana Buddhism considers meditation to be an altruistic act because it fosters wisdom and compassion. Sam Harris' Waking Up app is particularly great at taking meditation seriously; plus, the company has taken the Giving What We Can pledge.
Many charities and hospitals accept knitted and crocheted donations, and they usually prefer super-affordable acrylic. When I was learning to knit and crochet as a little kid, I donated a lot of preemie- and newborn-sized hats. The great thing about these crafts is that they can be either easy and meditative or creative and engaging.
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?
My best guess for something that most people can easily do that is still an ordinary day to day action but particularly high impact in this class is to get in touch with an elderly socially isolated relative. They will be very happy about your phone call.