CN Tower
Niagara Falls
Golden Gate
Mt. Fuji
Great Wall
Grand Canyon
Hoover Dam
Pyramids
Petra
Colosseum
Leaning Tower
Parthenon
Stonehenge
Big Ben
The Chunnel
Eiffel Tower
Mt. St.Michel
Empire St Bld

Alaska glacier
Tokyo Tower
Forbidden City
Sum'r Palace
LaBufadora
Bilbao Museo
BlarneyStone
Disney Paris

I´m developing this list as I go.

1.  Inside Look Up.  Outside Look Down.
Always look up inside cathedrals, churches, palaces, etc.  Some of the most intricate carved/painted artwork is on the ceiling.  Especially in Rome.  But outside, do the opposite:  look down to avoid the broken cobblestones and ubiquitous dog crap.  Especially in Paris.

2.  Dump, Tower, Explore
When you arrive in a city, dump your stuff off at your hotel/hostal, and immediately go out to find the nearest tower - if there´s one nearby - to get a good view of the city.  It´ll help you get your bearings so you can go explore.

3.  Leave your good stuff at home
When you´re traveling, especially long term, leave your favorite clothing, watch, etc. at home cause your stuff may get stolen, lost, worn out, or forgotten.  I left my favorite clothes, sunglasses and watch at home, replacing them with cheapies   Good thing, I´ve already lost 2 pens, a t-shirt, a digital photo media card, and my two sweatshirts are threading out at the seams.  Besides, fancy clothes usually attract pickpockets.  And it´s hard to negotiate good prices with your Rolex or Gucci bag showing.

4.  Learn the lingo, especially numbers
Forget your high school Spanish or French class.  Chances are, you´re speaking skills are rusty or nil.  And if your vocab was useful stuff like mine - "the pencil is under the book" - you´re screwed. Instead, learn the most often used phrases including "How much", "Where is the hotel/ bathroom/ cash machine/ restaurant" and "What time?" for trains, buses, opening and closing times.  Which leads to the most important, learn the numbers, especially 1-20, to tell the time and negotiate prices for cabs and souvenirs.  And it´s better to get people to quote prices in their native language as their accent may make their English difficult to understand.  For example, I thought one waiter said the price of a meal was "sixteen," but when I got the bill, it was "sixty" which he claimed he said.  They do sound similar.

5.  Proud to be an American - wisely
If you are, be wise about announcing you´re an American.  It´s not a good idea in the souvenir shops, taxis or streets unless you want inflated prices.  It´s perfectly fine in hotels and tourist cafes/restaurants.   I experienced the extreme in Egypt, where locals ALWAYS ask where you´re from.  When I said America in a souvenir shop or taxi, I got quadruple-plus prices, and twice almost got into a dangerous political debate about the US/Middle East conflict.  On the other hand, when I said America in a tourist hotel or restaurant, I got extremely prompt, attentive service and always seemed to get rooms or tables with great views.  So it depends, tho matters much less in Europe.  Also, frankly sometimes you´ll find you want to talk to other Americans just to talk in American English, so it helps to wave my Let´s Go guide book like a flag.  American travelers seem to carry it like the bible, while you can usually tell Brits and Aussies by their alternative Lonely Planets or Rough Guides.