I only get lunch out once a week...so I'm usually packing it. Either some sort of leftovers or a deli or tuna sandwich, a yogurt, some sort of carb (chips, pretzels, rice cake), fruit (apples, grapes, clementines, whatever we bought for the week), and something else - be it hummus, guacamole, kimchi (just made a bunch of cucumber kimchi the other night), and I should be eating more veggies.
I pack it all in an insulated bag with a freeze pack and into one of the panniers it goes!

I only like packing left overs if I have really good ones to choose from. Otherwise I prefer to save those for dinners. I've never had a container leak on me yet. I have these glass ones with an airtight lid that do really well. They are heavy, but I don't like reheating food in plastic.