20 Easy Ways to Help the Planet
In case you need a simple list that includes some good reminders and potentially new ideas!

Create Something New
A lot of times, just adding something to the world helps! Here are some ideas with hyperlinks to instructions to help you get started on just that!
Plant a garden, or even just a single plant— need help deciding on a plant? Try starting here!
Make seed bombs— it may be a bit messy, but it doesn't take long!
Make seed paper to plant— also a bit messy, but is easily giftable!
Build a birdhouse or bathouse— spend a day brushing up on your carpenter skills, or touch up a pre-made house with some decoration
Sew/decorate your own reusable shopping bags— limit your use of plastics with a customized bag!

Maintenance
Just keeping up with what's already there is a worthwhile pursuit.
Buy a plant, take care of it— there are easy maintenance options too, such as succulents!
Start using a reusable water bottle— it's popular among my college student friends to bring your own water bottle to meals. Here's some tips on picking one that's perfect for you!
Start shopping with reusable bags— If you'd rather not make your own bag, that's fine! There are plenty of bags you can buy once and reuse several times. For example, here's one SAMN has.

Healthy Removal
There are cases where taking things out of the world is for the best.
Pick up trash outside— you can do this by yourself or with others!
Turn off electric appliances you aren't actively using— can be good for the environment and your bill
Reduce pollution from shipping by buying local food— it might be healthier for you in more ways than one
Reduce pollution by walking, biking, or taking public transportation instead of driving— Imagine saving 4.2 billion gallons of gasoline due to people taking public transportation each year

Mental Practice
Physical action isn't the only action.
Work/Relax outside— just being in the environment helps build an appreciation for it, especially given how much it can help you
Meditate outside— Although it doesn't have a ton of direct benefits to the planet, it does benefit you, which will hopefully increase your appreciation for the outdoors and your activism for the environment.
Read about taking care of the planet— Knowledge is power! The more you know, the more you can do
Use a search engine that helps the planet— This includes ones that plant trees for your searches such as Ecosia, and ones that clean plastic out of the ocean such as OceanHero

Working Together
It'll take all of us to change the world, so why not start working together?
Give out seeds/plants or swap them— Makes it easier for people to contribute!
Go on a hike with friends/family— it has a few environmental positives on top of you getting to have a fun time outside
Organize a trash pickup— You can be strategic about where you do it too. For example, if you do it at the beach, then you can swim after!
Post what you do on social media— to help inspire others!
If you want to come up with your own idea, think about these categories. What can you add to the world, maintain in the world, remove from the world, learn about the world, or do to work together in order to help the world?