snacks for plane:
nuts
jerky
babybel cheese is wrapped in wax, so it travels well
quest bars

once beyond security, most airports have snacks like hard boiled eggs, veggies with dip, WATER or iced tea. I like the insides of a turkey wrap or deli chicken wrap.....

thai is rough- there is a LOT of sugar.
go for simple things like chicken skewers- skip the sugary peanut sauce. tom yum soup is great. coocnut based soups are generally fine. when in doubt ask for the meat or fish, with sauce on side. the veggies are awesome.

Copenhagen sounds like it will be easy- i have laways been fine in Europe.

DC is a breeze- I'm a native
We have everything- it is a very diverse population. In particular i love sushi places- we have loads- sashimi and miso soup is a perfect meal.
We also have a ton of steak houses.
in pubs, grab a burger and ask for salad instead instead of fries, then ditch the bun and eat it with a fork. also, may restaurants/ even pubs have crab cakes with no filler- just meat and egg/seasonings. a local chain Cylde's is a safe bet for any group- it's the default for bg groups with diverse palletes. nice, reasonable prices, very fun decor- each location has different theme.

when you are staying in hotels, if there is a mini fridge- i would keep things like deli meat, salad from grocery salad bar, cheese, bag of hard boiled eggs.....so you always have stuff. sometimes near rotiserrie chickens, they have smaller containers with just part of chicken or turkey. that would be ideal, since it is just for you.

have fun- that is a lot of travel!