Breathwork is simple and will do the trick. Plus, you can do it anywhere with zero preparation.
You need to learn abdominal breathing, cleansing breath, and bellows breath first.
Abdominal breathing is fairly simple, you want to learn to breathe deeply into the abdomen, as opposed to the chest. When you inhale, you should feel your navel rise and your abdomen distend. Once the abdomen is full, you can take a deeper breath by then allowing the chest to expand. This should be the way that you breathe naturally all the time if it isn't already.
The bellows breath is accomplished by inhaling deeply and then exhaling in a series of short, forceful breaths. This is easiest to accomplish at first by inhaling deeply into the abdomen and chest (as above) and then exhaling by saying "huh-huh-huh-huh-huh..." until your lungs are empty. About four or five times and you will feel full of energy, though possibly more if you are particularly lethargic at the time.
The cleansing breath is accomplished by sitting up straight, extending first the left arm out parallel to the ground, then bending the arm at the elbow and closing your left nostril with your forefinger. Inhale deeply through the right nostril, breathing in clean energy, and exhale through the right nostril while intending to breathe out all stagnant energy. You want to inhale from the nostril to the perineum, exhale from the right foot to the right nostril. After three breaths, switch sides - close right nostril in the same manner, breath in from nostril to perineum, breathe out from left foot to left nostril. After three breaths, release the right nostril and breathe in the same manner from nose to perineum, exhale from perineum to nose, three times.
For the warming breath, inhale to the perineum, work up a plug of saliva in the mouth, and swallow it on top of the breath. Hold for 8-10 seconds while focusing on the dan tien, about two inches below the navel and about two inches inside the abdomen. Raise the perineum (clench), hold the breath, then exhale through a tightened larnyx - like wheezing forcefully - while continuing to focus on the dan tien. After 8-10 breaths the spot will begin to get warm, and with practice you will be able to draw that warmth to a particular spot or throughout the body by imagining it moving.
The warming breath is not safe if you do not accomplish the cleansing breath at some point in the day first. Bellows and Cleansing breath are a great way to begin the day, so you should do them in the morning anyway, and cleansing breath again at night.