You won't pass much signage for this area but there are a few really cool sights to see here! Managed by the national park service, Mojave National Preserve has lava tubes, sand dunes, caves, and cool slot canyons to hike through, as well as some cool history! Typically this are would take a full day or two but the lava tube and Kelso Dunes can be done in a few hours. The lava tube is the closest thing to see from I15 which makes that specifically a very cool and easy stop.
For these locations, Google maps or Waze works great!
Mojave Lava Tube: About a 30 minute drive each way from the 15 freeway on Kelbaker Rd. The last few miles of it is dirt/gravel, high clearance is recommended but it's more of a groomed road with some washboard/annoying sections. I've seen Ford Mustangs and Toyota Prius' make it. From the parking area, it's a very short walk to the lava tube. To see this beam shining through the hole, you'll want to be there in the early to mid afternoon.
Kelso Dunes: Just under an hour drive from the 15 freeway on Kelbaker Rd. These are one of only 30 "booming" sand dunes which is a sound created when sliding down during certain atmospheric conditions. This is a large sand dune field which you can hike on and explore. It's not recommend to venture out on the dunes during extreme heat in the summer unless you have plenty of water and are well prepared. Other times of year when it's cooler, it's a great detour!