stay well away from the edge
One thing I really liked about the Grand Canyon is that there aren't fences everywhere. My daughter from last year:
http://i50.photobucket.com/albums/f346/Phil_in_CS/IMG_1398_zpse643c988.jpghttp://i50.photobucket.com/albums/f346/Phil_in_CS/IMG_1430_zps6f4bddfe.jpgThere are signs in a couple places indicating the rocks can be slippery, and in others that people have in fact died there. Even if you aren't in good shape, much of the south rim visitor area is handicapped accessible, and just walking along the rim is amazing. I know you've seen thousands of pictures, but I promise you that the reality of seeing it is completely different. The magnitude is overwhelming.
We didn't take the mule ride on Bright Angel, but my daughter and I hiked it for most of a day. I would not recommend that unless you're in good shape physically. The south rim is at 7700 feet elevation, and Bright Angel drops sharply, so coming out you might do 1500 vertical feet, and all of that at over one mile elevation. We didn't have any issues, but we saw people who were not having fun on the hike back up.
They do not sell any bottled water or soft drinks, or even any in plastic cups as they don't want the litter. Get a camelback or similar, or at least some nalgenes with belt clips. The have free water stations for you to refill your water. It's very dry there, and you will dehydrate much quicker than you think you will.
The first day we were there we stayed in Wilson and took the train ride to the South Rim visitor center. That was fun, and we had about 4 hours to wander about and decide what to do the other two days. The other nights we stayed in some small town about 10 minutes away, and if you have a national parks service pass you can ride the shuttle buses from the hotel to the park and not have to worry about parking at the park.