Heh, I could "see Las Vegas" in less than 24 hours.
Walk The Strip, view the lights and sights from the outside, then "get outta Dodge". I never liked the glitzy glamour of that place. All they want is your money.
Maybe others can suggest things to see or do in L.A., but driving over to Grand Canyon would be more memorable in my mind. (Can you guess I am more of an outdoors person?) I think the South Rim is more accessible.
Leave LV in the morning, and drive to Mt Whitney via Death Valley -- it's the lowest point in the continent, and the colors of the surrounding mountains are definitely something to see.
If you are ok with all the driving, the day before hiking Whitney, a drive up to the White Mountains and the Bristlecone Pine forest (at 10k' elevation) would give you more acclimatization. Not sure when it opens for summer, though. A call to the ranger station in Bishop would be good to check: (760) 873-2500
If you hired SMI guides, they could take you up the Mountaineer's Route, a shorter, more direct hike. It would be cool to go up that way and down the trail, but not sure the guides are allowed to descend that way. It's a major hike, and if you're just a sedentary tourist the two weeks before, Whitney can be a major challenge.
On your drive through Yosemite, leave early in the morning (or drive north to Bishop after your Whitney hike. Hit the Whoa Nellie Deli (at the Mobile fuel station on Hwy 120 at Hwy 395). On your way to San Francisco, it is WELL worth the side trip to descend into Yosemite Valley, view the valley and high granite walls, stop and walk up to the base of Yosemite Falls. Truly spectacular, particularly in May. It's about 3 hours drive from Yosemite to San Francisco.
