As in monopoly, something or someone that is seemingly unattainable but yet very desirable. Whether this is due to not being able to read or understand the person or object or whether the person or object is just too weird and indescribable (not to mention confusing and somewhat frustrating). If the thing you are describing is a person, it is someone who constantly surprises you and you are unable to rely on them due to their constant changes in behavior and attitude toward you. This also means you can go from loving them to hating them in a matter of seconds. If the thing you are describing is an object it is something that you cannot figure out its purpose or necessity to ever being invented.
Lindsay Lohan's strive to stay clean is a boardwalk just like Kate Gosselin trying to be normal.
Wearing belts when they are not attached to your pants via beltloops is a boardwalk- they aren't holding anything up!
Vince is a boardwalk...you never know WHICH Vince you are going to get.