Family Portraits

I am heading out to the Lake Mead area outside of Las Vegas today to take some shots of our family. We are going to an area which has a very narrow canyon with red-colored rock walls. I will be shooting in the middle of the day-the worst time to shoot, I know-and I need some tips for fill flash. I have rented a 600ex speedlite. Should I use the speedlite on camera or try to move it off camera? Also, any suggestions for lens selection? I definitely want to use the wide angle to get some dramatic shots. I am leaning towards the 70-200 just because it is my best lens.

Sorry for the dumb question. In the past I would just troll this forum to find someone else who had already asked my dumb question so as to hide my ignorance. With the new "100 post" rule, I have to ask my own dumb questions.

given your gear, for a landscape portrait, I would bring your 18-55 IS lens - it should be the most flexible

highspeed sync uses batteries quickly so make sure you have a fresh set

if it is a family outing that is about the family and not photography i would keep it simple - if it is about the pictures then a small shoot through umbrella would be perfect (but you are going to need a triggering device which you should be able to figure out a way to line of sight use your onboard 7D flash) - otherwise even a small modifier on the 600EX-RT while on the hotshoe would be good

highspeed sync dramatically lowers the flash distance so take a look at your Flash distance meter to make sure you are close enough