I have a couple of things I like to do when I find a copy of the dead-tree Mail on a train or bus.
The first is to take a biro - I always have one handy for just this kind of emergency situation - and draw a small Hitler moustache on Richard Littlejohn's photo byline. Just a small one will do. No glasses. No swasticka. No beard. Just a nice simple Hitler moustache. I do this because (a) it brings a lovely warm feeling of contentment and joy to my otherwise bland and tedious existence, and (b) I know it would really piss him off to think that (i) people take him seriously enough to hate him, (ii) someone is insulting him with a Nazi smear when clearly the biggest anti-semites in the world are lefties and trade unionists according to his bonkers loony nut-nut whoop-whoop logic, (iii) the moustache actually looks rather good, almost as if it belongs there, and (iv) it might be childish, silly, petulant and nonsensical, lacking originality, creativity and wit, but it's *still* the funniest thing that's ever been on his page in the history of the world.
The second thing I like to do is take a green felt-tip pen - I always have one handy for this kind of emergency situation - and draw a giant green cock-and-balls on the front-page splash. You can see the headline underneath still, of course, but the cock-and-balls is a neat and tidy way of demonstrating what you think of the lies, nonsense and general drivel beneath.
Sometimes, I like to expand this behaviour a little bit. I put a small cock-and-balls at the top of every single page, including the classified ads, all the way through to sport and onto the back page. I think this is my way of explaining that, while the front-page splash is a veritable tag-nut on Satan's very own anal ring, that doesn't mean that every single other page isn't just as evil, putrid, lying and disgraceful.
And then I pop the paper back where I found it, so that someone else might pick it up, and experience it for themselves.
NB This also works for the Evening Standard if you're in London, particularly when there's a front-page load of old nonsense about why Boris Johnson is god and evil slimy Ken wants to set fire to our children.
I have a slightly different approach to the Bristol Evening Post, the Mail-owned local title that comes out where I live. If I see a copy on a bus or train, I like to smother the inside pages with human vomit - I keep a small plastic bag of tramp vomit scraped up from the pavement outside Miss Millie's fried chicken for this emergency situation - and then leave the pristine front and back pages intact around the outside. That way, the reader will have a truly horrific experience in more ways than one when they open it up - firstly by the horrible stench of bile, disgusting appearance and rancid pages, and secondly because it's got sick on it.
As Neil Buchanan of Art Attack fame would say, "Try it yourselves!"