While HCHerb & tbarstow always have great ideas(cause they know the area's), you really should consider Zion NP, Cedar Breaks and possibly Bryce NP. You are gunna still have late light on your side.
You could also incorporate Red Rock Canyon(great 13mi loop west of Vegas) and Valley of Fire(north of Vegas).
While not discounting GC or DV, you could probably get more bang for you buck with a quicky up to UT. Also, let me throw in Kolob Res just east of St. George. These are all paved roads that may also lead to a more pleasant experience.