I would consider some non-amusement park trips as well. Here are a few fun trips we've taken with our kids during similar ages:
Monterey/Pacific Grove/Carmel California - Beaches to play on, tide pools to explore, the Monterey Aquarium, a helicopter ride along the coast, light houses, missions, hikes in Big Sur, a train ride through the redwoods. We go over spring break.
Our 2010 trip. Our 2008 trip. Our 2006 trip.
Colorado & Wyoming - Yellowstone (geysers, wildlife, waterfalls, hikes), Grand Tetons (stunning views, fun hikes), Rocky Mountain National Park (hikes, wildlife, mountain drives), Durango (gliding, steam train to Silverton, gold mines), Great Sand Dunes (fun in the sand).
Our 2009 trip. Our 2008 trip.
An All-Inclusive Beach Resort - We went to a place on the Mayan Riviera. Playing in the sand. Playing in the water, Ancient ruins. Swimming with dolphins.
Our 2007 trip.
Western Canada - Gorgeous mountains, beautiful gardens, rocky shorelines, wildlife.
Our 2007 trip.
A canoe or raft trip - We spent a few nights on the Buffalo river in Arkansas. The kids played in the rapids, cooked on the campfire, and explored nature.
Our 2007 trip. We also spent a couple of days canoeing around a local swamp (Caddo Lake) and camping on an island.
Our 2006 trip.
Kids are adaptable and can find fun in almost anyplace where they have some freedom to explore. Having variety in their vacations helps them experience the world in different ways. We love amusement parks, but there are a lot of other great options for vacations as well.