I pack my lunch sometimes, though not often since I work from home currently. But I still have to do so from time to time for other reasons.
Generally the best things I think to pack are portables like bananas, apples, boiled eggs, nuts, yogurt, pieces of cheese, crackers and/or various sandwiches. I also used to carry sardines at one point, I went through a phase.
Of course, many companies make it super easy for you to carry junk food like candy, cookies, and potato chips in individual serving packs, but I tend to avoid that most of the time.
When I worked in a place with a microwave and/or a fridge, or when I took things to school with me when I was in college, I was more likely to take microwavable meals (soup, pasta, lean cuisine, etc.) or left-overs.
I've never been devoted enough to carry things like fancy little bento boxes.
The most common problem with doing this is that sometimes you can be tempted to spend money even though you brought your lunch because fresher food or restaurant food looks more interesting.