As far as I know, there are no more beautiful blessings to offer the couple than those included in the Sheva Brachot recited under the chuppa as well as at the end of the wedding. If that's insufficient, for many couples, it's said multiple times throughout the following week, if they have Sheva Brachot meals. Look here for the text in Hebrew, translation and transliteration. Look, esp. at blessing #6 which calls the couple "beloved friends" and wishes them that they should be as happy which each other as Adam and Eve were (not directly, but it's metaphorically implied.)
As for what people wish friends, the most common saying I have heard the host tell guests and friends is "May we share numerous future simchot together." I think it's a great blessing of optimism for the future as well as the hope that each guest has cheerful future events and that the friendship will last long enough so that the host can return the "favor" by being present at the guests' events.