From Las Vegas go to Death Valley
. That is a must see - my favorite US park. And I have seen nearly all in the west. It is out of this world. Hoover Dam (as suggested) and Black Valley is worthwhile too - that is a short round trip of about 4 hours.
In Palm Springs, make sure you spend a day driving thru the Joshua Tree park
. Absolutely wonderful landscape. Then you can drive thru the valley to the Salton Sea
- very unusual, but smelly (H2S)
Lake Havasu is pretty dull - not worth it in my book. Lots of boaters.