The Great Global Cleanup is EarthDay.org's map of volunteer clean-ups, busiest around Earth Day (22 April) but open all year. You can add your own event, find one nearby, and download organising guides.
Honest caveat: it is a listing service; the quality of each event depends on the local organiser. Pair it with a data app (Marine Debris Tracker, Litterati or Clean Swell) so your effort produces evidence.
EarthDay.org is a 501(c)(3) (Employer Identification Number 13-3798288, Charity Navigator 4-star); donations at earthday.org/donate/ though note it spent more than it raised in 2024.
How to start: earthday.org/campaign/cleanup/