My current workplace has a policy where we are allowed to have a lot of flex time, we generally just have to ask the boss/check with project managers if we want to leave early and then make the time up later. My coworkers and I use this for everything from sporting events to concerts, we're just asked to not abuse it.
However, a lot of what I do in my free time is potentially controversial activism. I'd guess nobody would have a problem with it, but regardless I'd prefer to keep my politics seperate from my work. When I make a request to leave early/work an irregular schedule, my boss often reasonably asks what I want to leave early for. I'd prefer to not lie and get caught in a lie, though I know getting caught would be unlikely, and telling the truth could lead to a political debate I don't want. Up to this point I've been using euphemisms and half truths like, "I told my friends I could help them out at that time."
How can I best answer this question politely and without risking political tensions?