Five great modules you should play:
1. The Isle of Dread. It's a giant island that your players can explore as they see fit. There are a few quests, but really, they will make their own goals. I've run this a few times, and I don't ever recall anyone ever actually trying to finish it. It's a jungle island with dinosaurs, pirates, and zombies. So much fun!
2. Dwellers of the Forbidden City. Sneak into a ruined city and deal with the numerous groups in conflict with one another. Lots of adventuring sites. It's a free-wheeling game with a lot of fun stuff to encourage play. It has bullywugs, yuan-ti, bugbears, and mongrelmen, all of which are great fun.
3. The Lost City. A dungeon adventure in a desert. There's a pyramid. Break your way in, and then deal with the descendants of a lost people. It's got a lot of room to expand upon. Always good.
4. The Hidden Shrine of Tamoanchan. My favourite D&D module. Aztec ruin filled with poisonous gas. It's neat, because it punishes greed. If the players stop and explore every room, they die of poison gas inhalation. So, they have to figure the correct way out, all the while bypassing great treasures. Fun!
5. The Keep on the Borderlands. A classic adventure that so many players are familiar with. Probably the most famous adventure ever. It's vanilla, bland, and none of the NPCs have names. What makes it great, though, is that there's enough for you to run it comfortably, but plenty of room to come up with your own stuff. Also, it's a dungeon module where there are many ways in, and you can wander around and actually EXPLORE. Which is one of the big perks of RPGs, in my opinion.