Giving blood can be an exciting and rewarding way to spend a saturday afternoon. The blood loss will also make it easier for you to get drunk when you go out clubbing later in the evening.
Details:

The blood mobile moves around but you can usually find it outside of the Shinkong shopping mall by the MRT main station. The exact address is ZhongXiao W. rd. Sec. 1, no. 66. You don't need a reservation, just walk in the door and jam in a needle.