TIL that at least some airlines will give you a glass of milk if you ask (even though the milk is not on the menu except as an addition to tea/coffee). and unless you're lactose intolerant this is one of the healthiest consumables found on an airplane!
two other fun simple tricks:
1. you can ask for more than one drink or snack. if they offer pretzels or cookies you can say "I'll take both" or even "two cookies"
2. also, you can go to the back of the plane and just ask them for more of whatever free thing
it's possible some airlines will refuse this (especially cheapo ones although those often don't have free food anyway) but the fact that some clearly don't means they can hardly fault you for asking
as long as you're chill & polite about it I'm sure the flight attendants aren't bothered at all. they get lots of rude/mean people and they're probably just happy you aren't one of those
although maybe there's a quantity of asks at which even if you're asking politely they'll be like "uhh wtf this guy lol".