Windbreakers, little girl cardigans, and hoodies for the boys are a great place to start. For the adults, you'll want to cover your bases too.
Make sure you have the proper footwear for everyone to stay comfortable and not get tired. If you're an outdoorsy family, hiking boots are a great idea, and water shoes are essential if there's going to be water.
Investing in a baby carrier or wrap is the best way to keep your little one safe and your hands-free to enjoy the trip. It's a good idea to practice using your carrier or wrap before your trip, as they can be frustrating for first-timers.