Picnic by the Water
Head for the harbor or river, relax on a park bench or patch of grass, turn off your phone, and enjoy the spectacular scene. Whether it's sailboats or ocean liners, sea gulls or scullers, there's always something worth watching. My favorite spot is Sargent's Wharf, on the edge of the North End, but it's just one of thousands of pleasant spots.

Visit a Museum
Schedule your visit to take advantage of free or reduced admission at certain times. The USS Constitution Museum is free all the time; the Museum of Fine Arts is free after 4pm Wednesday; the Institute of Contemporary Art is free after 5pm Thursday; and the Children's Museum costs just $1 after 5pm on Friday.

Ride on a Swan
A Swan Boat, that is. The engagingly low-tech vessels ply the waters of the Public Garden lagoon at a stately pace, allowing an up-close view of the barely wild wildlife that flourishes in the heart of the city. For a family of four, a ride costs all of 8 bucks.

Take a Ranger-Led Tour
The National Park Service is such a good use of tax money. Free and cheap tours of historic attractions all over eastern Massachusetts elevate a visit to a park, a house, a neighborhood, or even a government installation (the Charlestown Navy Yard) from good to great.

Relish a Vicarious Thrill
Without so much as lacing up a sneaker, you can participate in the world-famous Boston Marathon. Stretch a little. Drink plenty of fluids. Claim a piece of sidewalk with a front-row view of the course. Cheer as the runners thunder past. Then put your feet up -- you must be exhausted.

Prowl Newbury Street
From the genteel Arlington Street end to the cutting-edge Massachusetts Avenue end, Boston's legendary shopping destination is 8 blocks of pure temptation: galleries, boutiques, jewelry and gift shops, and more. Fortunately, window-shopping is free.

Check Out a College Concert or Show
Countless student groups just want an attentive audience, and the free or minimal admission can pay off in the long run. Imagine the credit card commercial: "Ability to say you recognized the talent of [insert name of big star] in a student production? Priceless."