Many frozen fruits and vegetables are great for packed lunches. They often don't even need to be heated up. Just toss them in a container and they'll have defrosted by lunch. For fruits, mango and berries do especially well. For vegetables, peas, corn, edamame, and green beans are my favorites. If you have time and motivation, make a compound butter or vinaigrette to go with the vegetables or whip up a smoothie.
Have a can of beans? Blend it together with some oil and whatever herbs you have handy. Even better with some garlic or green onions. Or skip the blender and marinate the beans in your favorite salad dressing. Canned beans mashed with salad dressing makes a great sandwich spread.
Have a can of refried beans? Mix it with canned tomatoes, rotel, or salsa for a great bean dip or puree. For bonus points, add cumin and a bit of chili powder.
Have dry oatmeal? Throw it in a tupperware with plant milk or water and whatever you can scrounge up (consider frozen fruit, jam, dried fruit, nut butter, flax, or nuts). Not into sweet? Make savory oats instead (I like soy sauce, ginger, green onions, and hot sauce in my oatmeal). If you have access to a microwave at work, wait until lunch to cook. Best to use a large container for this and watch closely, as oatmeal can expand and overflow the container as it cooks.
Consider often-overlooked ingredients to round out your meal. Sun-dried tomatoes, pickles, olives, crystalized ginger, nori, and miso are usually in the back of my cabinets or fridge.
This last suggestion is a favorite of mine: soup in a bag. [Note: if Sandra Lee makes you pull out your hair, now is probably a good time to go visit someone else's blog.]
Soup in a Bag
frozen vegetables (peas, corn, or vegetable mix)
couscous, broken up angel hair pasta, or fideo noodles (uncooked)**
vegetarian bouillon powder/cube
Add a handful of vegetables to the bag. Add 1 tablespoon couscous or a handful of noodles. Shake a bit of garlic and ginger powder into the bag as well and finish it off with bouillon powder or a bouillon cube.
At lunch time, heat water. Dump the contents of the bag into a mug or bowl. Pour the hot (boiling is best, but not necessary) water over the soup ingredients. Wait 5 minutes for the pasta to absorb water. Stir and enjoy.
*There's no reason not to put the ingredients straight into a microwave-safe tupperware, but I find that mornings where there's no lunch food in the house are also mornings where there aren't any clean tupperware containers.
**You want noodles that will cook in 3-5 minutes. Anything longer (like regular spaghetti), and they won't soften with just boiling water.