I would suggest either Extinction Rebellion, which has an interesting open governance system with local chapters, and is “effective” in the sense of pissing people off enough to get itself on TV… or any local charity focused on a single achievable goal (sifting plastic pellets from a multi-ton dump at a rate of a few pounds per day, is a populist waste of resources; reforesting some area with native species, is direct and effective; and so on).