Finding people who are spontaneous is tough. Most seem to be driven by schedules, and feel it is unconventional to just go do something, or wander off on some adventure and see where it goes. I think some, perhaps subconsciously, feel that availability for spontaneous activities implies that they have no life.
Our culture seems to place a lot of importance on appearing important and in-demand, and I think it's getting worse with the cell phones, personal planners, blackberries, etc. If you can pencil someone in for a bike ride within less than a week, you must be some kind of loser or slacker.