If you've only got a day and you're continuing on I-40 you don't really have any choice. Drive up to the South Rim, wrassle yourself a place to park, and hike one of the trails part way down into the canyon. Try to save sufficient energy (and water) to hike back out, then proceed back to I-40 and onwards.
The North Rim is too far, and Lake Mead will fail to satisfy.
Hope that helps.