For $6, you get a 24 hour membership. This allows you to make as many trips as you want within 24 hours. Each trip must be 30 minutes or less, or you will be charged extra. If you're planning a long trip, this would seem to be a challenge. But it turns out this is not a huge problem. Just turn your bike in at another station before 30 minutes is up, wait two minutes, and check out another bike for another 30 minutes! Not sure this is how they intend it to be used, but it worked!
I picked up my first bike at City Hall Plaza, right next to the Government Center T station.