I would do three things.
Let's say we're starting in the South End. I would go for a run and run to Cambridge from the South End, down Mass Ave. End up at Life Alive. I usually order the Goddess or the Green Goddess bowl and a Coco Alive smoothie. It’s basically like a healthy version of a piña colada.
Visit this Central Square spot for juices, smoothies, and wholesome bowls with the most delicious, garlicky sauces you (and anyone else you run into that day) will ever have the opportunity to smell.
Then I would walk to the sailing pavilion and sail on the Charles. There are a few different sailing pavilions—for BU, MIT, and then Community Boating, which anyone can do. But basically it’d just be water sports for a few hours. And if you're running or walking, you should definitely do it along the water or the Charles.
Last weekend, I went to a Berklee jazz festival. That was really cool. It happens once a year, but Berklee has a lot of concerts if you sign up for their email list or check out their website.