Do the Grand Canyon OR Bryce Canyon, but not both. Since you're going from Palm Springs to Las Vegas I would lean heavily towards the Grand Canyon as it will be a shorter drive, [edit] and the Hoover Dam is on the way back to Las Vegas
Can you leave Palm Springs on the 2nd instead of the 3rd? That will give you a little more planning room. Or maybe your friends would be up for a quick road trip to Arizona and Nevada
The Hoover Dam- I would plan on that the day you go Grand Canyon to Las Vegas or the day you leave if you have time before your flight. It's only about a 30-40 minute drive from the strip. You could even hit it if there's a free half day on one of the confrence days.