The Back Bay is a large marine nature preserve in Newport Beach. There is a loop trail that allows you to explore the entire area. The entire trail is about 10 miles, but if you don't feel like exploring the whole thing you can easily walk portions of it. Guests staying at the Hyatt Regency have direct access to walk out to the loop trail.
Walking Balboa Island is the quintessential Newport Beach activity. Find parking along Marine Ave and then explore the island on foot. Window shop the boutiques and grab a famous "Balboa Bar" (as featured on The O.C.). Take the Balboa Island Ferry across Newport Harbor to the Newport Fun Zone (Fares: $1.25 for adults, $0.50 for children 5-11).