Don't throw away those orange and grapefruit peels...

throw some in a little dish and give them a twist during the day to release the wonderful citrus aroma.  Sniffing citrus scents helps release stress and can give you a boost in the afternoon if you're feeling tired. 

Tea tree oil also works wonders for an added boost in energy but many people don't like the medicinal smell.  I love it (but then I also love the smell of oil paint and WD-40)  and will often put just a drop on my left wrist so I can have occasional whiffs during the day.