Some close friends of ours have a portable dvd player they keep in their car for emergencies. If their son gets over tired or too wound up and they aren't able to just leave and remove him from the situation... they will pull out the dvd player and play thomas the train shows for him. I saw it and it really did work for him. He is 3.
They don't use it all the time, which I guess is why it does work and he loves and appreciates it so much. If they over used it and depended on it constantly, I can see how that might just get thrown aside like crayons or something.
Personally, for me, I don't go that route. Kids just get tired sometimes and you just have to leave. Most other parents will understand that your child just got to that point. But if it is not at "that point"... for me.... I just am trying to teach my daughter to listen and be patient and know that this 'quiet' time isn't forever and what is coming next. And that she needs to listen to listen, not because a movie was given to her if that makes sense at all?
Normally I notice with my daughter (who will be 3 soon), if I include her in everything, she is usually ok. I just make her a part of it, whatever we are doing. She feels more grown up and involved that way if I present the situation to her differently, rather than "shooshing" her. She loves to help with everything, so normally no matter what the situation, I can some how present it to her in a fashion that makes sense to her and makes her feel special and involved and important and somehow it just works.
Sometimes preparing her ahead of time for the day as well. Just letting her know what we are doing, and what is expected of her, that way there just is no question later when we get to that point. She was prepared ahead of time, so when she gets a warning as a reminder it sort of clicks. But I understand you are also saying last minute things. I agree that the more often you experience these things, the better prepared he will be of what to expect.
I have had to take her out to the car before depending on where or what is going on, sometimes you just have to. Luckily with her, she doesn't like the whole being "embarrassed" that she got in trouble so it normally does the trick. Boys are so different than girls though, so please know that I do realize that too.
I hope you get a lot of suggestions from other moms as well, and find something that works for you guys. Take care.