The Constitution and Bunker Hill are in Charlestown, not Boston proper, but are within walking distance of each other. Harvard is in Cambridge and the Freedom Trail is in Boston proper. If you want to do the Freedom Trail, stare and gape then staying close to Fanueil Hall/Quincy Market is the way to go. Lots of staring and gaping there, duck boats, trolley tours, and near T stations. This website may help in general:
https://www.nps.gov/bost/planyourvisit/maps.htmKeep in mind may places are termed "Boston" but really aren't. AirBnB should be much less expensive than hotels but please vet your neighborhoods carefully.