Have a need for lots of different sensors? Take a look at the Seeed Studio (seeedstudio.com) Grove devices. These plug into an Arduino shield using standardized wiring harnesses. They have a huge number of sensors to pick from, and most are quite inexpensive.
You'll need an Arduino to get started. What's nice is that the Grove system is all plug and play. Install your Arduino, buy a Grove Base Shield and the sensors you need, plug your sensors in, configure your Arduino, and start sensing.
One limitation to keep in mind. Currently, the Virtual Wiring system supports Arduino digital inputs and analog inputs for sensing (not I2C or the other serial protocols). Make sure the Grove sensor you want uses either digital and/or analog inputs - most do.