Microwaves. Not everyone has access to one at lunch, and some people just don't like them.*
Lots of foods taste great at room temperature. Many sandwiches, fruit, raw vegetables, crackers, salads, and nuts are all traditionally served room temperature. Other foods are often served cold, like summer soups, yogurt, smoothies, dips, and pasta and grain salads. And finally, there are foods that are usually served hot that just happen to taste pretty good at room temperature or cold, particularly pizza and pureed soups like tomato soup.
I know that I often don't feel satisfied until I've eaten something warm. If you have an electrical outlet available, you might consider getting an electric kettle. Electric kettles are pretty popular in England, but you don't see them as often in the U.S. They heat water very quickly with little energy use. With hot water, you can have tea, hot cocoa couscous, bulgur, and quick cooking noodles (like angel hair pasta). Even just a cup of tea following a cold meal is enough to make a difference for me.
Alternatively, think spicy. If you're feeling one kind of heat, you might not notice the lack of another kind.
If you're on the go or there's no electricity, you might consider getting a thermos or a food jar to keep things hot. Heat the thermos with boiling water, drain, and then add whatever hot dish you want. Chances are it will still be hot, or at least warm, at lunchtime.
Or think raw. Raw foods aren't meant to be heated in the first place. So revel in fresh fruits, vegetables, and sprouted grains and beans.
As far as keeping things cold, an ice pack should do. But really, most vegan foods should be fine at room temperature for a few hours, with the exception of rice and yogurt.
100 Lunch Foods That Don't Require Heating
1. tomato soup
2. pbj
3. pretzels
4. frozen fruit (it will defrost by lunchtime)
5. nuts
6. popcorn
7. fruit leather
8. leftover pizza
9. sesame noodles
10. steamed vegetables (w/vinaigrette or plain)
11. crackers
12. cole slaw
13. granola bar
14. yogurt
15. plant milk
16. bean dip (homemade or commercial)
17. fruit salad
19. pesto pasta
20. pickles
21. lemonade
22. avocado
23. trail mix
24. ants on a log
25. TLT
26. potato salad
27. tortilla and refried bean spirals w/salsa
28. nicoise salad
29. avocado and tomato sandwich
30. hummus and pita w/tomatoes
31. bagel with cream cheese or hummus
32. olives
34. iced tea
35. cinnamon toast
36. apple spread with peanut butter (core an apple, slice into wedges, apply peanut butter, reassemble apple)
37. veggies and dip (w/crackers!)
38. cereal (with or without milk)
39. tapioca pudding
40. chickpea salad sandwich
41. vegan sausage on a bun w/mustard
42. salsa and chips
44. tomato salad
45. roasted chickpeas
46. lettuce wraps
47. garlic bread
48. applesauce
49. seaweed salad
50. soy crisps
51. tapenade spread on French bread
52. juice
53. rice pudding
54. fruit cup
56. Waldorf salad
58. cucumber sandwiches
59. apple chips
60. gazpacho
62. vegan lunchmeat sandwich w/all the fixings
63. dried fruit
65. fruit-and-nut bar
66. packaged Indian meals
67. creamed spinach
68. edamame (throw in frozen, they'll defrost by lunch)
70. tofu satay w/peanut sauce
71. granola
73. marinated mushrooms
74. panzanella
76. three bean salad
77. equal parts tahini and agave nectar spread on a tortilla, rolled up, and sliced into spirals
78. broccoli slaw
79. brown rice w/peanut sauce (or curry sauce)
80. tofu (baked, fried, or aburaage) w/dipping sauce
81. cold baked sweet potato
82. marinated cucumbers
83. sushi rice with toppings
84. shredded carrots tossed with peanuts and raisins
85. banana chips
86. carrot sticks w/peanut butter
87. A roll sliced in half, spread with pesto, and filled with tomato slices
88. muffins (there's a great, filling peanut butter-oatmeal muffin in 500 Vegan Recipes)
89. seitan sub sandwich
90. applesauce
91. broccoli w/marinara for dipping
92. peanut butter and cracker sandwiches
93. dried figs stuffed with almonds
94. melon
95. guacamole (covered tightly with plastic) and chips
96. a soft pretzel w/mustard
97. ratatouille
98. cold roasted vegetables
99. baby spinach and mushrooms w/agave-mustard dressing
100. tabbouleh
*I think microwaves are perfectly safe to use if you microwave in appropriate materials (preferably glass. Old yogurt and margarine containers are not appropriate.). And they use less energy than most other heating methods. For the lazy, don't store any food you plan to reheat in a container you wouldn't put in the microwave. When you're hungry, you're more likely to throw caution to the wind. For the more prepared, cost-conscious, and waste-conscious, keep a bowl and plate at work so you can transfer food to a microwave-safe material.