Believe it or not, a DEEP BREATH and a few minutes of meditation can recharge you. Most people get lethargic also because they are dehydrated and don't drink enough water. Lots of sluggishness also happens because a lot of people have glycemic spikes from an imbalance of blood sugar. It is best not to have high sugar snacks as it only makes you crash. And of course taking a few minutes of fresh air is really helpful, too. Many of us work inside unhealthy buildings where the air does not recirculate - fresh air really does help.
Of course, getting a solid night's sleep every night will prevent mid-day slumps. We may not realize it but the QUALITY of sleep we get each night is really important. It is helpful to get a notepad before you go to bed and release all the thoughts that are floating around in your mind, so you don't obsess over things and you can truly lay your mind to rest to ensure your day ahead will be one full of pep.
Starting your day off with a good breakfast (not one high in sugar), is also helpful along with a multivitamin that is 'time released' - so you have something to sustain you all day long.