Let's play it by error.

"Let's see where the night takes us." is a really optimistic way to look at the future of your night time fun but it's difficult to make come to fruition. It's almost as it you're expecting positive change in a vacuum but I digress. Time to make more sense. Without at least some vague guidelines the most of the night will be spent in the parking lot of the first meeting place trying to figure out where to go and the destination might not being ideal. The initial idea of being spontaneous is to create a sense of mystery and free wheeling fun while being open to some possible "and one thing led to another" stories to tell others. At least thats my opinion. A few guidelines could help shape the night while not completely tying those involved to a schedule that is too strict.

• Communication: well duh, these days everyone has a cell phone and if they don't then why the hell are hanging out with the Amish. The night before the event people should check-in so people aren't left wondering "what the hell". If you've developed an overnight case of stigma let everyone know before the witching hour. Social net sights help too except everyone has their favorites and there is almost never a complete overlap.
• Population: The success of a spontaneous night out seems inversly proportional to number of people involved. This cause issues with transport and the "what do we do next" among other things. As far as I can tell there as a few remedies: appoint a leader or tie breaker, take fewer vehicles, or go to an area where everyone can walk; this last solution is contingent on where the people live (some cities just have a better nightlife for pub crawling).
• Have fun: you have to be able to enjoy the moment where every you are. That's what worked for me and I'm glad my friends got me outta the house.