I think claiming a health reason is bona fide, even if it is a white lie. Blame it on your back or whatever. "Oh, sorry, my back, my sciatica, my arthritis, my sports injury, etc. etc is kicking up" No one can argue with that, even if they don't believe you.
Another one I heard someone use that worked well in the situation was, "Oh, no thanks...that wasn't on my agenda for tonight." I guess it's another way to phrase it when you have other plans and don't want to do what they're trying to get you to do for the evening, the weekend, whatever.
Or just use the line about, "Oh sorry, I've got plans for that day...that weekend...that evening...whatever. Maybe your plans are to sit home and watch Netflix while eating fudge, but they don't need to know that.
Another good one if you're at all feeble in any way...maybe you've just been through some physical or emotional trauma or something...maybe you're just too tired because of being up at night with your kids or whatever, or from your job, is to just say, without explanation, "Sorry, but I'm just not up to it." Then if they keep asking, you just keep saying it. "Sorry, but I'm just not up to it."
Don't say too much, don't be apologetic, (the "sorry" is just a filler word) don't say something open-ended that will give them the impression that this is negotiable. Of course, if you want to help with whatever it is--and sometimes you will, of course...then it's a different story. We don't always want to get out of everything.