Jasmine or rose
Flowers are a global people pleaser, but their forms and hues aren't the only thing we find heartening. Jasmine and rose have historically been used in aromatherapy, and more recent research backs up their salutary reputation. In fact, a series of studies from the Human Emotions Laboratory at Rutgers University found that flowers in general are a "powerful positive emotion inducer," and that exposure to floral scents in particular can increase people's immediate associations with happy thoughts threefold.
All citrus smells are reputed to lift your mood, but the pure, bright scent of lemon oil seems to be particularly effective. Not only has it been shown to perk people up and reduce anxiety, other people also have a more positive impression of those wearing it. A recent study from Le Moyne College found that people associate lemony scents with femininity, cleanliness, and pleasantness.
When ingested, licorice can raise estrogen levels and blood pressure, and smelling it seems to have a faint pro-adrenal effect as well. As far back as the first century CE, physicians like the Greek writer Dioscorides were touting licorice for its ability to put patients in a mellow mood, and research from the Smell and Taste Treatment and Research Foundation has found that smelling licorice not only makes people happier, but makes them more physically aroused as well.
Lavender is an olfactory panacea: it's been shown to improve mood, soothe anxiety, increase sexual arousal, and even ease physical pain. Most importantly for the purposes of this story, however, recent work from the University of Miami School of Medicine showed that smelling lavender "had a significant transient effect of improving mood, making people feel more relaxed, and performing math computations faster." So not only will it make you happier, it just might give your Sudoku game an edge, too.