The Grand Canyon is awesome.  If you make it as close as Flagstaff, you should definitely check it out.  The drive time is only one and a half hours.  Once you get to the park, you can take in the sights as fast as thirty minutes, but you can do it more comprehensively in two to three hours.  Here's how to do it:

GETTING THERE

You can get to the Grand Canyon by taking I-40 W from Flagstaff to Hwy 64 N.  This route takes about an hour and a half and is an easy, direct route.  If you're a little more adventurous, you can take Hwy 180 W from Flagstaff to Hwy 64 N.  This route takes about the same time (some people think it takes less), and it's definitely more scenic.  If you want to take in the sights on both routes, you should take the I40 route in to the Canyon and the 180 route back to Flagstaff.  You'll catch great views of the San Francisco peaks by taking the 180 route back to Flagstaff.  It's worth it.

 WHAT TO DO

 Once you get your pass for the Canyon at the park gate, continue to drive into the park for about five miles.  After this, you'll come to a crossroad: going right takes you to the desert view of the Canyon and going straight takes you to the rim Trail and the Visitor's Center.  If you are really, really pressed for time, you can go right to get the quickest views of the Canyon.  Within only a few miles, there are two viewing areas.  Both give great views of the Canyon and are easy to access.  If you have more time, it's best to go to the parking for the Visitor's Center.  Once there, you can first check out the Visitor's Center to get oriented.  After this, you're ready for the South rim trail.  Simply follow the signs to the trail, and then enjoy the sights.  You can walk along the trail as far and as long as you like to take in the sights.  It's awe-inspiring.  It's awesome. 

 Enjoy your time.