black pepper; any kind of pepper actually.
ginger tea will stimulate as well.
they do this by stimulating the digestive system to produce digestive acids.
turmeric will work too.
all are used in treating anorexia.
just start using some of these spices, and others in your cooking.
a good tonic is a hot\warm tea made with turmeric, ginger, maple syrup or raw honey, and lemon; this is anti-inflammatory as well if you have muscle aches and pains etc.
do not go overboard and ease into their use... some of these spices, if you get hold of quality stuff, can have very powerful effects. Black pepper for instance, like other peppers, such as cayenne, serrano, jalapeño, etc., can have you sweating profusely even in cool temps at rest... it also works as a diuretic as well as stimulating fat metabolism, so be careful to eat potassium rich foods if you increase your use of, or use black pepper to excess.
I like black pepper specifically for its, antibacterial, antioxidant, manganese and vitamin k content, and the ability, when lightly cooked into dishes, to enhance the absorption of food nutrients that it is combined with. Note that it is best to use whole peppercorns, including the outer shells, grinding them just before consumption.