World Showcase is one of my favorite places in WDW. I will be heading there solo in one month and plan on spending 2 days there. Below are my recomendations.
1) See as much of the Entertainment at each country as you can. Ones I especialy enjoyed are The living statue in Italy, Off Kilter in Canada, The WS Players in England, The Acrobat's in China, and the Drums in Japan. But try to see as many as possible.
2) The attractions. The ride in Norway, Mexico, and the movies in Canada, China, and France.
3) Take time to explore each country. Don't forget to check out by the water, some of them have some surprises over their. Walk around, look in the shops, try to see the whole country. Make sure you go into the shops even if you don't plan on buying anything because the employees are from the country you are in and the stores are designed like they are from the host country, and the goods are unique as well. A few I really like are the shop's in England, Japan, and Germany. And you must visit the giant Troll in Norway. You'll know it when you see it. Remember, these countries are the closest thing to actually being in those countries. A great deal of effort went into making them authentic.
4) Sample foods from each country, ESPECIALLY THE PASTRY SHOP IN FRANCE. You may want to eat a nice lunch or dinner at one of the countries, or you could get a little sample from each country and let the family share so everyone can try something from each country.
5) Get there early when it opens up and spend the whole day there. It's kind of a circle, so if you make it around once and you have more time, just keep going and start again and try to see things you missed the first time around.
6) Though the American section is nice, we usually do not spend alot of time their for obvious reasons, but it is nice.
7) World Showcase has a ton of photo op's. have plenty of room on your card or plenty of film.
8) Don't miss Illuminations, the firework show at night.
Hope this helps, A good tip is to just rely on your map and Entertainment schedule, and see as much as you can without rushing.