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.

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. 

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.

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.

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.

Become a crisis counselor. 

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. 

Pros: 

• Immediate ability to help someone in need: delay times of as little as 10 minutes, including the time it takes you to get out your computer.
• Can be done from your house, directly from your computer.
• Very emotionally satisfying: There's not many places you can actively talk someone out of a panic attack, off the ledge, or just chat with someone who needs it.
• Awesome swag: Used in this instance to mean related clothing, coffee mugs, etc. My favorite hoodie is my 200 hour hoodie from CTL!
• Good training: certification as a crisis counselor is surprisingly good training for an online course. I've considered recommending it to EAs in general for this reason. (Also empathy building)
• Hours track directly: so reporting any volunteer service is very easy with this (good for resumes and similar things.)
• Good support network and chat on the platform.

Cons:

• Very emotionally demanding/frustrating: often times you'll be upset about how others handle their problems. I put this first because I struggle with this most of all, I want to just shake people and tell them how to get their life together. But as the CC that's not your role (you'll learn more about this in training.)
• Requires "decent" internet connection.
• "30 hour" course at the beginning: took me between 6 and 10 hours to complete, so it's not that bad.
• Background check (not crazy rigorous, but it is a text line)
• Not super effective, at most you can achieve a 3-1 time ratio (if you're really good)
• Emotionally draining, I list this again for a different reason. Sometimes talking to someone else about their problems can be overwhelming. Suicide, anxiety, depression, and abuse are common problems that you will confront directly.  Don't drown trying to save anyone. I've  been there, and you're too important for that. Trust me, seriously, you're too important for that.

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.