As noted by others, it is about a 3.5 hour drive from Phoenix to the Grand Canyon south rim by way of I-17 and Flagstaff. The South Rim is about 6800 feet elevation and it is expected to be partly cloudy this weekend so rim temperatures will probably be in the 70s to low 80s.
You can drive into the park (cost is $20 per car), but it gets really crowded in the summer and it can be hard to find a parking spot (not to mention the line to get into the park). Some of the overlooks (like Yaqi Point and Hermit's Rest) can only be accessed by shuttle bus while others (like Grandview) you can still drive to. If you have reservations at Maswick or Yavapai lodges, then you should have no problem finding parking. El Tovar, Bright Angel, Thunderbird and Kachina is more hit-n-miss (sometime you can find open spots hidden among the cabins to the left of Bright Angel Lodge).
Things to see and do:
Obviously there is the canyon itself. Try to get away from the most crowded spots near the lodges. Walking along any stretch of the rim trail between Bright Angel and Hermit's Rest is nice. Keep an eye out for the giant condors and the many smaller playful Ravens - both like to hang out along the rim near the lodges. Deer are also common. Sunset and sunrise are especially beautiful times to be near the rim.
There is a regular shuttle bus from Bright Angel Lodge to Hermits Rest that is very handy and runs along the rim with stops at many overlooks at regular intervals so you can get on and off as you like (unlike the other shuttle that connects the lodges, visitor center and eastern overlooks and is slow and difficult to navigate).
While looking at the canyon from the rim is nice, to really experience the canyon you need to get down inside of it a ways. I would not recommend trying to hike deep into the canyon during summer (it gets hotter the lower you go), but doing short hikes can be very rewarding. Depending on your level of fitness, I would recommend taking the Hermit's Trail down to Dripping Springs or the South Kaibab Trail as far as Cedar Ridge for good day hikes. You should be in good physical shape and always carry plenty of water. The rangers at the backcountry office near Maswick Lodge can give you good suggestions as well. You can sign up for mule rides into the canyon at Bright Angel Lodge, but it would probably be difficult to get last minute reservations on a popular weekend.
Aside from the canyon, there are many very interesting historic buildings along the south rim that are worth visiting. Many of them were designed in the early part of the 20th Century by Mary E. J. Coulter who was an architect for Fred Harvey and the Santa Fe railroad. Hopi House is a large gift shop built in 1905 and shaped like Hopi indian pueblo. Across from it is the famous El Tovar - one of the grandest hotels in the whole National Park system. Hermit's Rest is another Colter creation to the west of the central area that is built of stone and intended to look like an old hermit's cave built into the cliff. A few miles to the east (near the eastern border of the park) is Desert View Watchtower (a three story tall tower built to look like a prehistoric Anasazi ruin). The layout of the canyon is different here (starts to open up to the east) so it is worth visiting.
For food, I would avoid the overcrowded and low quality dining hall in the Bright Angel Lodge. My personal favorite is the Arizona Room Grill (kinda hidden on the east end of Bright Angel Lodge - excellent BBQ and steaks). Maswick has a good, fast, and relatively cheap cafeteria. El Tovar has a grand diningroom that is definitely worth eating at if you have the money and reservations.
Things to see and do nearby:
Flagstaff is a very interesting historic town in the mountains that you will pass through on your way to the canyon. It has a historic downtown area with old buildings and neat shops and cafes. The Museum of Northern Arizona (just north of town on your way to the canyon) is an excellent museum with lots of info about the area and there is also an interesting museum at Lowell Observatory overlooking the downtown area (this is where planet Pluto was discovered in 1930).
The north rim of the Grand Canyon is also very nice in summer. It is at a higher elevation and is much cooler and less crowded than the south rim (there is only one small lodge there). However, it is much further away - about a 4-5 hour drive from Flagstaff.
Staying at one of the lodges in the park near the rim is the best way to go by far, but you need to have had reservations long in advance (the odds of getting a room at the last moment are very low). If you don't have a room at the canyon, there is the collection of chain hotels at Tuysan (just outside the park boundry), but I would almost recommend staying in Flagstaff as an alternative.
Don't forget to bring your camera, something to carry water in, comfortable shoes or hiking boots (regardless of if you plan to hike or not), sunscreen, a broad-brimed hat and a light sweater or jacket for night would not be bad.
Relax, marvel, enjoy. It is one of the greatest places on earth.