A B.O.B. trailer would probably serve your needs. They're not cheap, but less than a new bike and it would take up less space. The B.O.B. corners great but if you will be hauling more than the recommended 75lb limit, it becomes difficult to control. A two wheel trailer is more stable under heavy loads but won't corner as well. A bike with B.O.B. is quite nimble, can handle the moderate loads you describe, easy to carry up stairs. B.O.B. trailers are light weight and you can put one on your bike or take it off in seconds.
The Zipcars are worthless. You don't need Zipcar. Good for short trips, but bicycles are better for short trips. I'd ride a bus before hiring a Zipcar. Rent a car for long road trips, not for short jaunts.
Last edited by qmsdc15; 03-25-10 at 04:11 PM.