As a Bostonian I can't recommend the Duck Tour enough, it's a blast even for us living here.. the "Captains" make the amphibious tour of Boston informative and fun. You really do not need to do trip excursions in Boston.. Boston is a very walkable city. There will be cabs outside the Black Falcon Pier..you can take it to the QuicyMarket and pick up a Duck Tour there or walk to the North End and do the Freedom Trail (Paul Revere's home), eat at the fabulous italian restaurants or wander into Mike's Pastry or one of the other famous shops like Modern. As for Portland...it's a walkable city..the Old Port is charming...and we like the City Market (away from the Old Port) which is actually a local market with one of the booths selling the StoneWall Jams... You may want to go on a whale watch..if they offer it..
As for Sydney..I know nothing about it.. except there is a Fort.. If you'd care to write to me I'll send you moe info on Boston especially