All of the advice you have received is both correct and useful and should be heeded. I will add that Bright Angel CG and Phantom Ranch are just about 10 miles and approximately 5000' below the rim. This is not a backpack for kids or adults that are not familiar with the GC. I have spent over 120 nights below the rim on rafting trips and we hike extensively. If you want an enjoyable day hike I suggest taking the Bright Angel trail down to what is called First Water, it is approximately 1 mile. Second Water is 2 miles. Either hike will be enjoyable and mostly stress free. Indian Gardens is 5 miles and, again, only for the experienced hiker.It is also the only place you can camp on the way to Phantom.Hope you have a great time.
RichH