Cities. Sometimes a source of claustrophobia, more often a place of busy, active people doing and interesting things.
Boston & Cambridge, MA: During the summers of 2002 & 2003, I was able to learn Boston from the view of 11 science and math teachers from Southern Africa who had never been to the USA. So, my viewpoint might be a bit skewed. Here's a sampling of what they liked best: the Science Museum (especially the Computer Clubhouse), the New England Aquarium (particularly the IMAX/3D films - "so cool that they were ice"), the ocean, the green majesty of the White Mountains (especially Gorham, NH), the Boston Harbor Islands, the T, shopping, taking the Chinatown bus for a day in New York City, and MIT. Excellent North End restaurant: Artu Ristorante. Canoeing or kayaking on the Charles River. The Apple Store.
Hong Kong: Eat at the many scrumptious restaurants. Ride the tram to the Peak for a cup of coffee at the coffeehouse at the top. Get out of town: visit the islands. Don't miss the race track.
London: In London, the theatre is a bargain (compared to the US). If you go to the Tower of London, can you see if Bart Simpson is still in the window above the front gates?
Paris: Paris is probably easiest if you like the French and French culture. Once you become a dedicated Francophile (or are able to laugh at the quirks of Paris), there are amazing gems in Paris: drinking hot chocolate and eating a fresh croissant at many corner cafes for breakfast (e.g. Cafe de la Mairie), attending a concert in Sainte-Chapelle, touring the amazing Musee de l'Armee (especially the sections dedicated to the WWII Resistance, WWI, and the Holocast), walking in Montmartre, dining in amazing restaurants at midnight (visit www.fodors.com for good recs), finding antiques in a flea market, eating chocolate crepes on the Eiffel Tower, visiting the Musee D'Orsay, and catching a performance in the Opera Garnier.
San Franciso: The amazing views from the Golden Gate Bridge - well worth a nice walk. Riding the cable cars. The Exploratorium. Eating tea leaf salad at a Burmese restaurant, such as the Burma Super Star Restaurant.
Stanford: Drive up Palm Drive near sunset, watch baseball in Sunken Diamond at night, visit the "Gates of Hell" in the Rodin Sculpture Garden. Watch old movies at the Stanford Theatre in Palo Alto.
Toronto: Oh Canada! Toronto is a beautiful city. Toronto is an easy city to walk in - and you can get to museums, shops, restaurants and charming tree-lined residential areas on foot. The Hockey Hall of Fame is the home to the Stanley Cup (when it isn't on tour) - the fans are nearly as much fun to watch as the exhibits. The Skydome is a nice place to catch a Blue Jays baseball game. And the bookstores are filled with titles printed in the U.K. that you cannot buy in US bookstores. But perhaps my favorite of all: many high caliber restaurants in a diversity of styles. Really, the food is stunning, and the seafood particularly fresh and well-prepared in many restaurants - check out Fodor's Critics Choice for tips to good ones.