10:43 a.m. on January 10, 2012 (EST)
What are you crazy? Forgive me but May will be very hot, especially down in the canyon. Well over 100 degree's in the shade! Stay on the rim, for that matter go to the north rim not the south one. The north rim is 1000 feet higher and much cooler.
If you do go into the canyon start as early as possible before sunrise and get to your designated camspot and sit in the shade by the creek. Don't venture to far from water and carry and drink at least a gallon every couple hours. When you are ready to hike out start as early as possible. It used to take me an average of 5 hours to hike out when I lived in the canyon during the winters from 1983-2003 and I was in tip top shape. Its a 5000 foot elevation change between the bottom and the rim. It takes less than a couple hours to hike down, but for every foot in its harder to hike back out.
Best months to hike the Grand Canyon are from late October to early April. I spent much of my time there in November to March below the rim. I only went back to the rim to resupply once a month.
Mid April to late October the inner canyon temps can be well over 100 degree's, not counting passive solar temps(reflected and stored heat of and in the rocks).
If you must go down be sure that you head my warnings, the canyon is not the place for the un-informed. Many people succome to heat stroke and death in the canyon every year.
5-9 days? I would not recommend going into the canyon, where as Florida has what 90% humidity, the inner canyon has about 10%. Stay along the rims. There is plenty to see between Hermits Rest on the west rim to Desert View on the east rim on the south side. One the North Rim there is also lots to see and do without the long hike to the bottom in 100 degree heat.
From the South Rim its 9 miles and 5000 feet down to Bright Angel and Phantom Ranch on either the South Kaibab or Bright Angel Trails. The B.A. Trail has a water stops every 1 1/2 miles from the rim to Indian Gardens but no water except a small creek you have to purify and then the Colorado for the about 5 miles from I.G. to BA Camp and Phantom Ranch.
From the North Rim to the Colorado River is 14 miles with a 6000 foot drop in elevation. There are places to stop for water (Roaring Springs, Cottonwood Camp, Ribbon Falls, Phanton Canyon and the whole Bright Angel Creek from Cottonwood camp to the Colorado,BA Camp and Phantom Ranch).
Water along the other trails in the inner canyon is sparse in some side canyons with seasonal water flow, sometimes you may find water right where the trail crosses a side canyon, in others you may have to walk up or downstream a long way to find water on the surface.
Maybe if you can afford it and theres space, take a mule ride down and back. They leave the rims early in the mornings. There may not be any open permits in late May for either hiking/camping or taking the mules. Its usually a 3-4 month waiting period to get a permit. If you have'nt gotten one yet, go to the Grand Canyon NPS site www.nps.gov/grca/index.htm and look for the permits reservation information. Now is the time to apply for any permits. Sometimes if you have the time to wait you can get standby permits but summer (April to October) is the busiest time of the year and its harder to get them. In winter I have waited as much as 4 days for a standby permit. Hiking/camping fee's are about $10 per permit and $5 a night in the canyon. It costs about $20 a night to stay in a rim campground (2003 prices). Cheaper if you have a car and can drive outside the park entrances from either rim and camp free in the Nat forest.
One the South rim for you being frist timesr, I would recommend the BA Trail as it has theeasiest places to get water. The South Kaibab has no water from the rim to the BA Camp. In fact none of the many south rim trails have much water along them at all. The only north rim trail that does is the North Kaibab mentioned above.
Be sure to take some method of water purification to use the natural water sources in the inner canyon if you do manage to get permits. Drink often and use of electrolites is best. I like instant gatorade and use 2 three liter Camelbaks plus two one liter water bottles.
Wildlife to see is Big Horn Sheep, Rattlesnakes, Scorpions, Ravens,Deer, Spotted Skunks, Ringtailed Cats, Ground Squirrels and Chipmonks and the occasional long distance hiker.
Tempertures rise about 10-20 degrees per 1000 feet into the canyon, depending on the time of day. There is little shade in the inner canyon. Indian Gardens, BA, Phantom and Cottonwood Camp have planted Cottonwoods. Otherwise the only shade comes from big stones. The inner canyon is a desert. Tempertures are simular to what the daily temps are in Phoenix and Tucson on any given day. Right now its 43 here in Tucson, so its at least that in the canyon at the Colorado River area.
And you are coming from Florida, elevation could be a factor for oxygen content compared to what your'e used to. At the South Rim its about 7500-8000 feet above sea level and about 9000 feet on the North Rim. There will be no snow anywhere but compared to Florida it will probably seem colder especially at night when it drops to the 40s on the north rim and 50-60s on the south rim.