Ideal For Ages 5 and up

Telescopes, binoculars, and even astronomers are on hand to inspire young scientists to cast their gaze skyward at Boston University’s free weekly Public Open Night. Both bedtimes (in the spring and summer, the hourlong sessions begin at 8:30) and telescope-using abilities make this better for older kids, who will be dazzled by the complex wonders of the moon, planets, and countless stars. Call ahead to make sure weather conditions haven’t forced a cancellation. 617-353-2630; bu.edu/astronomy

Soak Up Sun and History on Castle Island

Ideal For All ages

Castle Island offers the chance to explore Boston’s inner harbor from multiple points of view. The island is connected by causeways to South Boston, allowing for easy walking or biking access to Carson Beach, M Street Beach, and other spots along Dorchester Bay. Fort Independence, a pentagonal defense structure, marks the area as a military guard post dating back to 1634. Free tours are available from Memorial Day through Columbus Day. 617-268-8870; mass.gov/dcr 

Explore Nature at the Arnold Arboretum

Ideal For All ages

The arboretum’s 281 acres in Jamaica Plain and Roslindale are a wonderland of plant life, from the springtime lilacs to the deep colors of autumn trees. In addition to simple nature walks, family-friendly activities abound. Kids ages 3 and up can borrow free Discovery Packs containing materials to explore the park as a Living Museum or Science Lab. Families can also play Wildlife Bingo on print-at-home cards or do a season-specific “photo hunt,” searching for and photographing plants described in free downloadable lists. 617-524-1718; arboretum.harvard.edu

Imagine a Sailor’s Life Aboard the USS Constitution

Ideal For History and military buffs ages 8 and up

Go below decks on the world’s oldest commissioned warship that’s still afloat today. A US Navy sailor will lead your free 30-minute tour of the 44-gun frigate, which first set sail in 1797. Learn about life at sea during the Barbary Wars and the War of 1812, and discover the heroic reason this wooden ship was given its famous nickname, “Old Ironsides.” Visitors 18 and older must present a government-issued photo ID to board the ship, docked in Charlestown. 617-242-5601; www.history.navy.mil

Don’t Forget the Tourist Favorites

Ideal For All ages

The grassy Boston Common, the country’s first public park, and the more formal neighboring Public Garden come alive in springtime. In the Garden, admire the blossoms and play on and snap a photo at the iconic Make Way for Ducklings sculpture. On the Common, relax at the large playground next to the Frog Pond, which opens as a free splash pool in late June. Younger kids will ooh and aah when they see the golden dome of the Massachusetts State House next door, while older kids might like to step inside for a free tour (high schoolers can even train to become volunteer guides). Frog Pond: 617-635-2120; bostonfrogpond.com / State House: 617-727-3676; www.sec.state.ma.us

Take in the View at the Bunker Hill Monument

Ideal For All ages, notably 8- to 12-year-olds studying the American Revolution

The site of the first major battle of the American Revolution will appeal to both history buffs and those up for a physical challenge: Visitors can climb the 294 steps of Charlestown’s obelisk-shaped monument to take in a sweeping view of the famous battle site and the neighborhood that now surrounds it. The Battle of Bunker Hill Museum, also free, features a large diorama, complete with sounds, lights, and narration. 617-242-5641; nps.gov/bost

MARK YOUR SUMMER CALENDARS

Revere Beach National Sand Sculpting Festival July 18-20

Remarkable sand creations emerge each year on America’s historic first public beach. A free Interactive Kid Zone is packed with hands-on activities, including sand-sculpting lessons. 781-902-9742; reverebeachpartnership.com

Lowell Folk Festival July 25-27

Folk, bluegrass, and ethnic music spills into the streets at this lively gathering, and free family activities like beanbag toss and “box hockey” games up the fun quotient. 978-970-5000; lowellfolkfestival.org

Downtown Plymouth Waterfront Festival August 23

Listen to live music on two stages, with children’s entertainers featured on the main one, watch the exhilarating Ducky Dash race in Brewster Gardens (purchase a $2 rubber duck to enter), or admire the classic cars on display at the Motor Heads Cruisin’ Car Show during this daylong event. 508-830-1620; plymouthwaterfrontfestival.com

FROM MODERN ART TO STUFFED MONKEYS

Visit these museums without paying a penny.

The Discovery Museums Acton

Free admission first Friday of the month, October-June, 4:30-8:30 p.m. 978-264-4200; discoverymuseums.org

Fuller Craft Museum Brockton

Free admission Thursdays, 5-9 p.m. 508-588-6000; fullercraft.org

Harvard Museum of Natural History Harvard Square

Free admission to Massachusetts residents Sundays (year-round), 9 a.m.-noon, and Wednesdays (September-May), 3-5 p.m. 617-495-3045; hmnh.harvard.edu

Institute of Contemporary Art Seaport District

Free admission Thursdays, 5-9 p.m. 617-478-3100; icaboston.org

STORY TIME, CRAFT PROJECTS, AND MORE

Many retailers offer free activities for children. A sampling:

Kids Workshops at Home Depot Multiple Locations

First Saturday of the month, materials included; first come, first served. 800-466-3337; workshops.homedepot.com

Build and Grow Classes at Lowe’s Multiple Locations

Select Saturdays, materials included, registration required. 800-445-6937; lowesbuildandgrow.com

Smaland at IKEA Stoughton

Supervision provided at drop-off play space for potty-trained kids from 37 to 50 inches tall.

888-888-4532; ikea.com

Jeremy Lyons Singalong at Stellabella Toys Cambridge

Saturdays, 11 a.m. 617-491-6290; stellabellatoys.com

Music at Henry Bear’s Park Multiple Locations

Thursdays, 11 a.m., singalong with Jeff Jam, Arlington, 781-646-9400; one Friday per month, 5-6 p.m., music, Cambridge, 617-547-8424, and Brookline, 617-264-2422. henrybear.com.

Family Activities at Barefoot Books Concord

Times and dates vary; music, story times, infant play groups, and some crafts. 978-369-1770; barefootbooks.com

Storyteller Susan Lenoe at Andover Bookstore Andover

Fridays, 10 a.m. 978-475-0143; hugobookstores.com

Story Time at Newtonville Books Newton Centre

Tuesdays and Saturdays, 11 a.m. 617-244-6619; newtonvillebooks.com

Story Hour at Porter Square Books Cambridge

Wednesdays, 11 a.m. 617-491-2220; portersquarebooks.com

Story Time at Brookline Booksmith Brookline

Third Saturday and last Sunday of each month, 10:30 a.m. 617-566-6660; brooklinebooksmith-shop.com

Holly Lebowitz Rossi lives in Arlington with her husband and toddler son. Send comments to magazine@globe.com .