Difficult one that - at first I was gonna say, why not stick with the 18-200 for the entire trip and get use to your camera via the one lens.
But then, if you really want to learn, then the 35mm will be of good use too especially if you're willing to swap to it indoors.
However, I have the SB400 and have soon learned it's really only good for indoor flash use up closer. Wish I could afford a better flash unit though.
You probably don't want to use a flash for the parade(s) as you want to capture the raw colours so to speak.