If you feel yourself getting tired at work, sometimes just going outside to get some fresh air and some sunlight will do the trick. Take a walk during lunch -- sunlight and physical activity activate parts of your brain associated with memory and mental energy. And you want both of those!
Eat a (healthy) snack
It's probably not a good idea to turn to chocolate bars (which also contain caffeine) for a quick high, since that sugar rush will leave you down in the dumps 10 minutes later. Low blood sugar will make you feel drowsy, so if you're feeling sluggish, find nutrient-rich foods to snack on. Carrots, oranges, nuts, avocado, or beans are good options to have on hand.
Just be careful with lunch. If you eat too large a meal, you risk falling into the dreaded food coma while you digest, instead of diving back into the daily hustle.