It might be enough, or far from enough, depending on your interestes and what special programs are on for the day of your visit. The Seaport has a constantly-changing program of exhibits, walks, talks boat rides and children's events. Here's a link that will give you more ideas--it's the daily activity schedule for Spring 2013
As for Mystic Pizza--it's got a wild variety of quite-good pizzas, but you won't meet Julia Roberts there, or even see much about the movie. In other words, don't go for the show, go for the pizza!?