The class kit already has many different sensors, mainly for measuring "environmental conditions". You could start by using any of them individually, to see what each can tell you (which is what some of the assignments are asking you to do, so you have that experience and knowledge). Besides learning about the sensors, and the Arduino, you'll have learned enough that you can then start combining many sensors into one Arduino sketch. (Remember that adding many libraries to your sketch will eat up memory, and may limit your programs.)

I think that a better question is "What question do I want to answer?" I find that I'm more likely to get through a tough learning curve if I'm trying to do something that I *want* to do. When I run into difficulty along the way, I remind myself that I want to get through this, to find an answer, to have a useful script, to have a database I can search, or to have graphs for a presentation. 

Do you have questions that you want to answer? If not, consider these;

• How does watering the lawn affect local humidity? 
• What happens to Temp/Humidity when the sun goes down? Or when it comes up?
• How fast does the temperature rise in the morning, or fall at night? 
• Can I use a barometric sensor to sense opening/closing a door?
• How can I control a relay, turning on/off at a certain temperature?
• Does the number of students in the classroom affect Temp/Humidity in the room?
• Does Carbon Dioxide build up in our classroom with the door closed? 
• Does Carbon Dioxide build up in my bedroom if I close the door?
• Does vacuuming the house actually put more dust into the air?
• How long does it take for new paint/carpet smell to go away?

You can answer any/all of these questions with the parts in the class kit. But maybe you have your own questions. Consider telling a story about how information from certain sensors, and collecting their data, will help answer your question.

• Think about what you want to know, then think about which sensors can collect the information you need.
• Consider the timing interval, if you want to graph the information (in seconds, minutes, hours?)
• Do you want to add a visual or audible alert if certain conditions are met?
• Do you need a real-time clock, or can you just use rough time by counting the number of times you went through the loop?
• Will all this fit on a small breadboard, a long breadboard, or do I need more than one?

With the information above, you're ready to go set up your breadboard, and start working on your sketch. 

Or, take a look at other interesting products like these tiny sensor combinations, and think about what it offers, and how you could build something like it. Or this smart home sprinkler system, and think about what it would take to build something like this. When you have learned about interfacing a few sensors, you'll be ready to add even more to your next project.