Phnom Penh Cambodia Highlight
Some other suggestions, although not necessary - Travel pillow, chewing gum (especially for those who have ear problems during flight), book and/or ipod and one change of clothing (air lines do lose luggage occasionally)
The trolley tour of Boston, offered as an optional, is the best way to see the entire city. Take it at your own pace, but I would suggest stopping off at the USS Constitution and Fenway.
Christmas Markets in Vienna are great. Buy some mulled wine (Gluhwein in German)which is served hot and is quite spicy. The mug it comes in is yours to keep - a great souvenir -but if you don't want it return it and get your €2 deposit back (Pfand).