Best free things to do in Baltimore?

The Walters Art Museum is free and fantastic. Have lunch at Gertrude's.

The Baltimore Harbor is free. There are plenty of eateries, plus not too expensive water taxi rides to the other side of the Harbor with eateries and shops.

Walk around the Inner Harbor, Fells Point, Little Italy. The Walters Art Museum is awesome!

Certainly go to the Inner Harbor to hang out. There are several museums close by that are inexpensive or free. Pick up a local brochure when you get there to plan a day.

Hi,
Baltimore has a lot of history. If you love history you should start off by visiting Enoch Pratt Free Library located in the heart of Baltimore on Cathedral St. (Mt. Vernon District). The library opened in 1933 with a grand chamber and artwork.
Mt Vernon has been designated as the National Landmark and Historical district. A visit to Washington Monument is amazing as well. You can't miss it as it sits in the middle of Charles St. surrounded by old cobblestone roads and sidewalks.
Mt. Vernon is one of the city’s oldest neighborhoods and originally was home to the most wealthy and fashionable families. The name derives from the Mount Vernon home of George Washington; the original Washington Monument, a massive pillar commenced in 1815 to commemorate the first president of the United States, is the defining feature of the neighborhood.
Although mainly residential, Mount Vernon is home to a mix of institutions, schools and museums. If you get hungry, visit Mount Vernon Marketplace. The market has several bars and restaurants.
A visit to the inner harbor is nice as well. The harbor is surrounded by museums and restaurants as well. The Walters Art Museum, Maryland Science Center, Reginald F. Lewis Museum, Jewish Museum of Maryland. You can take a water taxi from Harborplace and tour the different Stops (Federal Hill-Montgomery, Locust Point, Fells Point, and Harbor East). All have various bars and restaurants and consist of amazing water views and a lot of walking!
You can also walk to these areas as well but make sure you have good walking shoes on. Pack a lunch and eat on any of the above referenced piers. When my family visited from Germany, we visited the super market on Charles and Saratoga Streets and grabed lunch items then walked south on Charles street to the Harbor to eat. You can also picnic on Federal Hill across the street from the Harbor with amazing views!!
For hiking, I suggest Gwynn Falls Trailhead 1, Jones Falls Trail, and Druid Hill Park (The Maryland Zoo in Baltimore) are all nice.
Lastly, if you are still in Baltimore on a Sunday, I suggest visiting the Baltimore Farmers' Market & Bazaar located under the Jones Falls Express Way between Hoilday and Saratoga Streets.

Federal Hill Park has a great view of the harbor and a cool neighborhood to explore. Inner Harbor is fun to walk around. Fells Point has some funky shops and great bars.