I can't think of too many items that are truly "fast food" that fit the bill for "healthy", at least in the context of someone looking to make a significant change to their body composition. Food prep is definitely the big picture, but it helps to have other options.
Here are some of my go-to options when I'm feeling lazy or just sick of cooking and cleaning all of the damn time.
Chicken Panang Curry: I get this from my local Thai kitchen and it is not a bad option at all, and it is my go-to meal when I want a delicious lunch that I don't need to cook or clean. This is just boneless/skinless chicken, red and green peppers, carrots, white rice covered in a sauce made from spices, coconut milk and a little peanut butter. I will sometimes eat this five days in a row. I love it. It is not my favorite item on the menu, which would be Pad Thai, but it is what I almost always order because Pad Thai ain't getting me closer to my goals.
I will also order off of the menu or put a special order quite often.
I will sometimes order the panang curry without rice and with extra veggies. Now its low carb.
The menu at the bar I work at is full of crap, but I can pick and choose items to make a special order that ends up being a pretty solid meal. You could do a lot worse with a patty of grilled ground beef topped with grilled onions, mushrooms and peppers with a side of steamed vegetables. Grilled chicken with veggies works too, and most bars will have it.
Another item that is not on the menu is simple scrambled eggs from McDonalds. It has been a while since I got these, but they sell them for like a buck an egg. They are pretty crappy, but again a comparatively good option when in a pinch, when travelling, etc. Just a scrambled egg.
McDonalds will also serve you meat patties sandwiched between lettuce. I've never done this, but I've witnessed it.
Ask the wait staff. When I sit down for a meal or order carry out, I look for something that is basically meat and vegetables. If I don't see it, I ask about it. I love steak and it almost always comes with a potato, but I will usually ask for double-veggies instead (not that potatoes are bad, per se, I'm just keeping my carbs low right now).
Remember, most restaurants still want picky people's money. Tell them what you want. It also helps to have some "go-to" options known in advance, like my Chicken Panang Curry. That way I don't even need to look at the menu and be tempted with Pad Thai. I just order the good stuff.