My Aunt Nancy, Uncle Randy, cousin Katie, and her boyfriend Jordan were in town from Claysburg, PA (pop. 1350) this weekend. I tried to show them a wide variety of things. Based on that experience and previous visits from my parents, I wanted to make a list of ten things to remember for future guests. Here it goes:
1.) Take Them on The Subway – Fast and cheap ($2 to anywhere) it's the best way to see more. Most tourists look at the entrances like steps to hell. There is normally construction and route changes on the weekend, so they are right to be scared. As a seasoned New Yorker, you can add a ton of benefit to your guest visit just by preventing them from getting lost underground and saving a bundle on taxis. – MTA.info 
2.) Walk, walk, walk. – The best thing to see in this town is the street and the people. Tell your kin to pack comfortable shoes and expect to be sore when they go home. Be prepared for the "Now I know why New Yorkers are so thin!" comments and walk them until they fall down. They will sleep better if they are tired anyways and the most memorable experience of the trip will probably be something unplanned that you stroll by on the way from one site to another. 
3.) Grand Central – Since the refurbishing finished in TK YEAR, Grand Central is one of the most impressive sites in town, but unlike the Empire State Building, there are no lines. It makes a great midday stop. There is a food court for a fast, inexpensive lunch and relatively clean bathrooms. You can hop off the Green Line in between the Museum Mile and the Seaport for a nice wrap or maybe even a quick bit of mac-n-cheese at the bar at Michael Jordan's.
4.) Dive Bars – The sticker shock of a trip to NYC can melt the fanny pack right off of a bus tourist. Take them to a dive bar so they can see how we can afford to go out every night. Rudy's is close the theater district. The Subway Inn is right beside Bloomingdales. A cheap pitcher will be a welcome site to someone who has been marched all day.
– Rudy's Free hot dogs and popcorn.
– Subway Inn 
5. ) Breakfast – Start the day with a decent foundation. If you can get there before 9AM, try Cafe Habana (where Lenny Kravitz once stalked in a video). If not, do the Silver Spur. Breakfast is inexpensive so it should be at a sit down place and enjoyed before the long day.
6.) Traveling Exhibition – If possible, take them to something that is only around for a limited time, like the Bodies Exhibit. The sense of urgency adds value, plus it gets you off the couch to see something that you might regret missing.
7. ) Chinatown Knock-offs – They all want ugly cheap purses that look like ugly expensive purses (I hate labels, it's just my thing). Don't deny them. Go to the corner of Canal and Mott. Look for a woman with a walkie talkie and tell her you want purses. If you look harmless, she will take you up some stairs to a hidden store with all the knock-off goodness anyone could want.
8.) Ethnic Food – I make everyone who visits try some type of food that they have never had before. Sushi is normally the big winner and does not get many complaints as long as saki is also on the table. If their plates looked picked at, but not empty, I take them to the 4th Street Bar for wings immediately afterward. My goal for all this is not to expand anyone's mind, but rather to get a free plate of sushi.
9.) Circle Line – Not everyone can walk all day. The Circle Line is a good alternative. The tour is interesting for tourists and New Yorkers alike. Talk the half tour, the one that circumnavigates the island is just too long. – The Circle Line 
10.) Walk Across the Brooklyn Bridge – Always a favorite, the good ole' Brooklyn Bridge. The wires create interesting perspectives, there are great views, and it's always good people watching. Just at the other end is Grimaldi's, arguably the best slice in town. The water taxi provides an alternative way home or to another site. This one is always a winner. Also noteworthy: Off off Broadway, Central Park, the Esplanade, the Staten Island Ferry, Statue of Liberty at night (shorter lines), Chumley's speakeasy, Katz's Deli, and the Roosevelt Island Tram. These and the list above are all my fallbacks. New Yorkers and visitors alike, please leave your favorites in the comments.
… Site of the Day: A good followup to my shredder piece, Rob at Cockeyed.com gets a credit card sent to another address with a torn up application.