Depending on the time of day you're traveling, the GPS will probably take you on different routes. From Los Angeles, you would most likely just hop on I-10 and spend the entire drive on an interstate. From San Diego, you might be routed different ways. Due to what I am assuming was rush hour traffic in Riverside County, the GPS took us up to Temecula on I-15 and then we exited and spent most of the trip driving on fairly isolated roads (State Road 79, 371, and 74) until we got to Palm Desert. There really wasn't an issue with this but there were not very many places to stop and get gas and cell phone service was spotty.
We started our visit at the southern entrance of the park and if you start here, I would recommend stopping for gas in either Palm Desert or Indio, because there is no where to stop after that.
Driving home, the GPS took us a different way through more populated areas of Riverside County (State Road 62, I-10, I-215), and it felt like a much more direct route. The trip to J-tree took a little over 3 hours and the trip home took just under 2.5 hours.
I do not recommend doing this as a day trip
We woke up at 5 am and got home around 9 pm. Obviously 5.5 hours of that was driving. So it was a long day. I think if you are already closer to J-tree you could probably justify a very worthwhile day trip, but with the amount of driving we did, it felt like we should have spent the night somewhere. I was conflicted about spending 2 days because I had something planned in San Diego we could only do on Thursday and we wouldn't have been able to do that if we'd stayed overnight. We also didn't have camping gear since we didn't want to haul all that with us from GA, so camping would not have been an option. There are plenty of vintage looking roadside motels in the towns nearby. I also looked on AirBnb and saw plenty of options for lodging there as well.
If you do a day trip, plan on a very long, jam packed day
As stated, if you're only there for a day you have to make sure it's worth your while. Maybe it's worth you while just to drive around and look at the main attractions. I wanted to get some hikes in though, so we had to have enough time to do that. I would recommend being in the park by 8 or 9 am to make sure you have a full day before sunset (depending on the time of year and when the sun sets).
If you do a day trip, make sure the weather is tolerable
The temperature the day we were at J-Tree got up to about 78 degrees, so we were relatively comfortable hiking in the early afternoon which was when we started a 4 mile hike. If it had been a warmer season, I'm not sure hiking at 2 pm when it's 98 degrees (or hotter) would have been a good idea. If you're there in warm weather, it's recommended to hike early in the morning and then chill out somewhere in the heat of the day. But if you're only there for a day, you don't have time to chill.
The cooler weather allowed us to have a full day doing whatever we wanted, when we wanted, so I'm sure that is why the park is most popular in the winter months.
Bring food, water, and emergency gear
You can only access running water at the visitor centers (and some of the campgrounds,I think), so either bring a large "little league" style water container full of water and ice or bottled water. Since it wasn't terribly hot the day we were there, our bottled water stayed a nice temperature sitting in the car. We drank at least 16 ounces of water every hour we were outside. I had never experienced "dry heat" before and I felt noticeable thirsty very often (we went to AZ last year but I felt J-tree was much dryer). I also used Nuun water enhancers.
Similarly, you need to bring food with you, unless you want to drive in and out of the park to go to restaurants. We brought almond butter and jelly sandwiches, Clif Bars, bananas, tangerines, dried fruit, granola, and chips. Whatever amount of food you need to sustain energy levels, bring it.
We had no cell reception in the park. I also didn't see any rangers out and about while hiking. I brought my basic hiking emergency which includes a compass, whistle, fire starter (there are regulations about fires in the park), tarp, etc.
Sunscreen should be a given, but I chose to just wear long sleeves and a hat the whole day. I feel better when I don't have to stress about re-applying sunscreen all day. We were fine hiking in running shoes, although I saw a lot of people wearing Chaco type sandals. Be careful if wearing sandals. You know, there are a lot of "pokey things", as James calls them, in the desert. I forgot to bring a jacket, which was a mistake because it got very cold and windy in the evening and I had to sit in the car to watch the sun set.
Be sure to pay your entrance fee
Have a great trip!
Visiting Joshua Tree is something I've wanted to do since I was a kid. It exceeded anything I could have ever imagined. I hope you have a great trip to Joshua Tree as well!!