I would speak/tweet/post/argue. The marches themselves are only for the spirits of the marchers - and that is not nothing.
I personally don't tweet. I think it's a total waste of time, especially after Trump started tweeting about covfefe, or whatever. I certainly discuss - and at least may have made one Leaver pause for thought.
It's a question of what can you do? I have thought often, why didn't people do something in 1930s Germany? Now I begin to know - partly because people don't have a full picture of what's happening, and partly because we don't know what actions are going to be effective. I think it all helps. Take whatever action speaks to you.