I coludnt agree more....about the canyon being underated..IMO its as good as the rock, i worked at the resort there for 2 stints of 6months and did the canyon numerous times, its different each time, cliche yes, if you do go try and get a guided tour with one of the rangers early morning just as the sun comes up, also avoid doing in the middle of summer as mid day temp around the rim is usally round the 40s!! i was out there when it was 44 and it truley is unbearable!!
Last year there was alot of rain out there and everything was green instead of red and it was great, also waterfalls go off the sides of the canyon and from the basin (called the garden of eden) makes for some amazing photos.
Oh make sure youre reasonably fit to do the rim walk, as theres a pretty steep incline to start with which is the only really hard bit, but once youre at the top its a easy walk, i dont think its worth going all the way out there unless you walk the rim, the greek walk is ok but the rim will blow you away (literally, dont stand near the edges as there really isnt any gaurdrails)
As for the resort...well they have a monopoly out there and if youre booked with atp you dont have a choice as to youre accomodation.
You could go for a sunset camal ride on the day you arrive at the rock.