- The I2C is designed to have one single master, with many slaves. You only need SDA and SCL (both will pull-up resistors) and you can attach many slaves. Every slave reacts if its ID is transmitted on the bus. But if some sensors use 5V and others use 3.3V, you need extra hardware for level shifting.
- Simple sensors like sensors with analog outputs (MMA7361 accelerometer), or sensors with a single digital line (DHT11, temperature + humidity) can be connected to the Arduino directly.
If you want to use the MMA7361 accelerometer, you need 3.3V again. But you need 3 analog inputs.
The digital version MMA7455 is also running at 3.3V and can be attached to the I2C bus. So you use less pins, but you need an example or a library to get the digital communication working.
These MMA... series accelerometers are cheap. Many people use the BMA... series accelerometer which are more expensive.
The whole tinkerkit idea is to have a shield, and with connectors and cables you can attach sensors. It is very fast (just click the shield and cables) and the result is very nice and clean looking.
But to test or develop something by yourself, you could solder the sensors yourself and learn about them along the way.