I'm in my early 30s and getting married for the second time. Having my father walk me down the aisle at my first wedding was one of the biggest joys of my life, but is it still appropriate for him to do the honor now that I'm remarrying?

By all means! As long as your father is willing and able, there are no etiquette rules against his escorting you to the altar once more. And you certainly wouldn’t want to offend him by not asking him to do so.