The Boston public garden is right beside the commons and is one of the most beautiful places in Boston. Parking can be a little tricky in the city so I'd recommend finding a spot using the Spot Hero app or parking in the Boston common garage.
The North End is famous for its Italian restaurants and almost any restaurant usually has good food. Our personal favorites are Panza, Giacomo's and Tratoria Ill Panino. Modern bakery and Mike's pastries have some of the best cannolis too. Would again recommend finding a garage for parking via google or use the Spot hero app.
Great outdoor place with loads of sculptures.
You can check out Harvard University and also explore local shops and restaurants. Would recommend going to LA Burdick for some of their hot chocolate.
Castle island is an old decommissioned fort that's still preseved. Pleasure Bay right next to it is a protected beach meaning kids can play in the waters without you worrying about them getting swept away by waves. There's also a nice walkway over the water from pleasure bay to castle island if you're in the mood for it.
For the more adventurous spirit, there's kayaking on the Charles which is simply wonderful in the summer. You can see the skyline from the river and go by the esplanade as well.