Weather stations are one of the most common projects around hobbyists. You actually don’t need much to build one. For instance Andre Sentosa put together one of its versions. He took Arduino boards for the brains, then attached a bunch of various sensors including DHT11 humidity and temperature, BMP185 barometric sensor, rain drop sensor and soil humidity.
With a bit of code he was able to log data to serial monitor. There are still plenty room for more sensors like LDR, wind speed and so on to make it more advanced. IF you are thinking on getting serious, you should also think about remote data transfer and convenient logging.