Afternoons in that part of the world can be incredibly bumpy, so bumpy it's miserable. I've been up there some afternoons and couldn't get above the mechanical and convective turbulence because of oxygen regulations. Arrive at GCN in the morning, hike around and spend the night at a lodge on the rim. Leave the next morning and fly over the top - or before you arrive if you wish. If you fly over the top take a hand held GPS and put all the waypoints in there, it makes flying the narrow corridors so much easier. Flying when it is cool will also give you better climb to 11500ft. Remember a lot of rental planes are a bit tired and will not give you the performance in the book. You should also be aware of the "gotchas" when at high density altitudes. Not sure Nellis will care too much, but LA centre will know if you bust the SFRA and will ask you to call when you land.