This might be too late, but....
You are not going to GC or DV. Too far.
1. Red Rock Canyon on the west side of town. It is a 13mi. loop of fantastic views.
2. Valley of Fire State Park just north of Vegas. Well eat up 3-4 hours.
3. Out of VofF link up to Northshore Rd to Hoover Dam. Park just before the dam and walk up to the Bypass Bridge
Enjoy Vegas, tell my $$$ hello.