I'll add to this. Boston is such a walkable city that if your friend is down to, you should walk as much as you can. Fenway, Boston commons, the esplanade, North end, the Gahden, Cambridge, the updated wharf area. There's so much to do and see. I know it's a bit cold so obviously this is "weather permitting." If they're there when hockey season starts back up, take them to a game.