I could find ways to buy a monthly pass or load stored value, but could not find a way to buy a 7 day pass (even though it is advertised here. Any ideas how to buy a 7 day pass?
You can buy 7-day CharlieTickets and CharlieCards in person at MBTA subway stations. I personally recommend the 7-day Zone 1A LinkPass. It's printed on a paper CharlieTicket, but it will additionally allow you to use the commuter rail within Zone 1A and the Charlestown Ferry for the same price as the CharlieCard.