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Sedona - Park the Expy and rent a Jeep Wrangler for a day at 'Farabees'. This is not a guided tour with someone else driving. This is you driving a late model, lifted Jeep with BFG's. They even supply the cooler. The rates are reasonable. Check out their website. I found them by accident when I did your same trip 2 years ago. Sedona is beautiful and the kids would have a blast on the Jeep trails.
Vegas - If this is the first time just take in the sights and madness of the strip. The Tournament of Kings show at Excaliber is fun, comes with dinner and is not as pricey as some of the other shows in Vegas. If you need a break from the strip take a drive to Red Rock Canyon, 1/2 an hour, beautiful area and weather permitting its a good place for the kids to run around. If you want a break from the heat and want to stay outdoors head up to Mt. Charleston, 45 minutes, you can drive to over 10,000 feet high. There is a lodge and resturaunt up there.
Grand Canyon - All I can say is awesome view and keep your eye on the Demon Seed! People fall into the canyon every year.
Have a safe trip, Im sure you will be comfortable in the Expy.