Start with the basics. Toiletries are essential for any overnight trip. Pack a small bag with a travel size version of your favorite regularly used items such as, shampoo, conditioner, soap, deodorant, toothbrush and toothpaste.
Check the weather. The weather varies greatly in southern California during winter and temperatures can range from 60 to 80 degrees Fahrenheit in one afternoon.
Pack jeans and T-shirts, and bring a sweatshirt for the evening if the weather is going to be between 75 and 65 degrees. If weather is forecast to be below 65 degrees, pack a heavier jacket along with the sweatshirt. If the forecast predicts rain, purchase and pack rain ponchos ahead of time; they can be quite expensive in the park.
Pack clothes that can be layered. The temperature in the park will vary throughout the day and it is important to pack layers so that they can be peeled off or added when temperature changes.
Wear or pack tennis shoes, they will be good for walking miles through the park. Comfortable tennis shoes will suffice in any weather.
