It really depends on what your options are. For years, my habit has been to make extras when I make dinner and pack meals into Tupperware and freeze it. Then I just grab a frozen meal in the morning to take to work. I usually don't put it in the fridge--I just let it slowly defrost over the course of the morning and then microwave it for a minute or two to heat it up at lunch time.

Sometimes, I'll just have a main dish frozen and ready to go, so I add a container of frozen veg. I just fill up a container with whatever is in the freezer(broccoli, spinach, cauliflower, stir fry mixed veg, etc.), sprinkle a little salt on and add some butter to the container. Then I microwave that at lunch. Or maybe I'll make a quick salad and take along some dressing to add.

Some other easy options are meat and cheese roll ups, salami, cheese cubes, etc. Or a big salad with added leftover chicken, steak, etc.

Low carb drinks depends on if you want something sweet (and can tolerate the artificial sweeteners) or not-sweet. I mostly always drink water when I'm at work during the day and that's what I have with my lunch. If I'm at home, I have homemade unsweetened "tea" (the kinds that don't really have any real tea in them like ginger-turmeric, orange, lemon verbena, because I don't like unsweet real tea), soda water, or occasionally some Crystal Light. But I try to keep the artificial sweeteners to a minimum, so I don't have them in drinks every day.