Being spontaneous isn't something you can plan. That's just a contradiction. You can plan fake spontaniety though. Say you've been to wherever your date is planned beforehand. If there was some unusual thing there that you could be spontaneous about, you'd know about it in advance and could figure out what you want to say. Like say there is some kind of show to attend at the place you're going. When you get there and walk by it, you'll have something great planned to say that will seem spontaneous.
Of course, that's a lot of extra work for something that isn't hard in the first place. If you really wanna be spontaneous, just do the first thing you think of, right when you think of it.