I look forward to going back this April
Four years ago, we were driving from a stay in Las Vegas and making our way to the North Rim of GC. We went to Red Rock Canyon and Valley of Fire. That was in August. We enjoyed both enough that if we want to get away from all the glitz of Vegas, these are two very enjoyable destinations to visit to be outdoors and do a bit of hiking. I'm in much better shape fitness-wise now and I'm sure April will be quite a bit more comfortable than August.
We considered going to Death Valley, but I feel that would absolutely require an overnight stay in the area and we have a lot of plans for Vegas since the one time we stayed there it was a short stay, we met up with family and didn't do much exploring or see any shows.
As far as when to visit, I'm not sure. My husband loves sunrises. I love sunsets. But I'm sure there's a happy medium somewhere there. If you go, enjoy! I'm from the East Coast and don't have opportunities often to see these types of landscapes.