Sunny Buns wrote:... I remember some big sand dunes somewhere around Yuma or El Centro, just off the freeway where you could rent an ATV by the hour. That was fun 30 years ago........ Bet you can still do it....
Imperial Dunes.
Not sure of the drive time to any of these from Yuma, but if you can work it in, this area has some great hiking opportunities ...
Anza Borrego Desert State Park is the biggest state park in the country. Not sure its exact size, but I've been told by a volunteer of many decades that the Wikipedia page sizing is usually too small due to land donations that continue to increase its size--roughly 700,000 acres now. Popular with the off-road crowd, but there are opportunities to hike. Check weekends for big off-road events in the area, because it can be slammed with people who give off-roading a bad name.
Nearby (to ABDSP) Carrizo Mountain is also popular with the off-road crowd, but it's closed to them (officially--some miscreants work around that by finding alternate routes around the gate, or by going in after the closing date but before BLM officials have actually locked the gate shut) starting January 1st or each year through the end of June (IIRC).
The San Diego County high point, Hot Springs Mountain, is located on Los Coyotes Indian Reservation. They have camping on the reservation, and you could bag the high point. Great views from up there. Off-road travel is now allowed up there, but most folks don't know that because it was closed for years to them.
Mt. San Jacinto is a great hike with wonderful views of the desert. You can shorten the hike by taking the tram up, or hike it all the way from the desert floor below (near Palm Springs). It's also possible to hike it from the other side at Idyllwild, but a longer drive from Yuma.
If you want to climb some rocks, head for Joshua Tree National Park.