Hydroquinone is the gold standard to fade sun spots. You can find 2% in over the counter products (i.e. Ambi, Nadinola, Alpha Hydrox) or 4% in prescription creams. Other ingredients that have lightening abilites (that don´t seem to live up to hydroquinone though) include Vitamin C, kojic acid, licorice I believe and I´m sure many more. Some people use lemon juice, but I don´t know if it may make your skin more sensitive to the sun or not.
But I´d go with 2% hydroquinone and a sunscreen with sufficient UVA-protection. But any lightening agent will need a couple of weeks to work, so don´t expect fast results.