Mount Desert Island's (MDI) big draw is Acadia National Park. Acadia may be dwarfed by Yellowstone and Glacier, but it's a giant when it comes to dramatic scenery -- its rocky peaks drop through thick evergreen shoulders clear to the deep blue. And there's no shortage of fun ways to keep the gang moving. There's hiking for kids of all ages, from flat seaside paths and gradual ascents to thrill-seeking steep stuff and 45 miles of groomed carriage paths for biking (rent tag-a-longs and trailers for the wee ones). Get wet at the lone sand beach (or the significantly warmer Echo Lake). Steal a waterman's view of the rugged coast by kayak, nature cruise, or ferry ride. Gawk at all sorts of wildlife, from harbor seals to sea ducks and peregrine falcons to bald eagles.
The park is open all year, but most of the island's support services shut down from late October through May when the weather is harsh and cold. You can make the most of Acadia's outdoor opportunities from June through mid-October. Black flies can be pesky into June and mosquitoes are more likely if the season's been wet. Tourists are thickest in July and August. MDI and Acadia are quieter in September and early October, and no less spectacular, thanks to the fall foliage show.
Summer is prime time for both tourists and fog. Temperatures average in the 70s, but may drop into the 60s or spike to the 90s. Be ready for wet or cool weather; you'll want a wind jacket and sweater or fleece for cooler nights, boat outings, and windy summits. Get more weather info.
Though well worth it, it is a bit of chore to reach MDI, which is 50 miles (about an hour) from the nearest international airport in Bangor (BGR). MDI is 161 miles (about three-and-a-half hours) from Portland International Jetport (PWM), and 264 miles (about six hours) from Boston's Logan Airport (BOS). Unless you want to take the pricey option of flying from from Boston to the tiny Bar Harbor-Hancock County Airport (BHB), a stone's throw from the park, renting a car is essential. Train service on Amtrak's Downeaster gets you only as far north as Portland; Greyhound bus lines go to Bangor.
A car gives you flexibility to hop around the island on a kid-driven timetable, but you'll need a car pass to enter the park May through October. You can pick up a seven-day car pass ($20 high season) at several points on the island, including park entrances and on the Village Green at Bar Harbor. Note that during summer, the island and the park are thick with cars, and parking for many hot spots often fills up by mid- to late-morning. In this case, you can opt for the free propane-powered shuttle bus system called the Island Explorer. The extensive network takes passengers throughout the park (individual riders must still pay park entrance fees) and all over the island from late June to Columbus Day. You can ditch your car whenever you like to get to dinner, shops, hiking, and more. Stick to the bus entirely, and you can purchase individual passes for $5 each.