A few suggestions for you:
恭喜發財 gung1 hei2 faat3 choi4 新年快樂 san1 nin4 faai3 lok6
-- All purpose happy new year.
身體健康 san1 tai2 gin6 hong1 龍馬精神 lung4 ma2 jing1 san4
-- Mostly for older people (and health-conscious people), wishing them good health.
快高長大 faai3 gou1 jeung2 daai6 學業進步 hok6 yip6 jeun3 bou6
-- For children and students respectively, wishing them grow tall quickly, and make good progress in school.
步步高升 bou6 bou6 gou1 sing1
-- Suitable for working people, business associates, etc., wishing them advancement in their career.
喜氣洋洋 hei2 hei3 yeung4 yeung4
-- Also a general purpose wish for happiness. Since this is the year of the sheep, some people choose this one because of the 洋洋, a play on the word 羊 "sheep".