Most attractions in El Segundo are within walking or biking distance, and there are plenty of bike rental shops around town. If you plan to visit downtown Los Angeles, it's recommended to take the Metro Rail (approx $3.50) or an Uber/Lyft (approx $25-$30 one way) due to the distance and traffic.
If you have more time, consider visiting nearby Manhattan Beach or Hermosa Beach for some great food and nightlife. For a shorter trip, you can remove the day trip to downtown LA and spend more time exploring El Segundo's local attractions. Make sure to check the weather forecast beforehand and pack accordingly for beach and hiking days.