I remember reading parenting advice, once upon a time (likely before I had kids, when there was time to read manuals for advice) that said that parents are often too quick to say "no." I've solved that issue with the terms: "possibly," "perhaps later," "now is not the best time," "remind me tomorrow," "ask your father," "not right now," "give me a few minutes to consider..." etc. When the kids press for a straight answer my reply is usually something to this effect: "I can't tell you yes or no right now. I don't want to lie to you and have you be disappointed. I will give you yes or no when I know what dad has planned/what the weather is/what tomorrow will bring etc."
Truthfully, in my own mind I'm often saying "no way" but DS and DD haven't quite figured that out yet.
When DS was small my standard answer was usually "later." So much so that he became quite confused about timelines. He would often ask "when is later?" Tell me, how do you define that? He got so that he believed a day consisted of morning, afternoon, later, evening, night. But sometimes "later" came before afternoon, or after evening. It was all very confusing for him. The seasons also got muddled up -- spring, summer, later, fall, winter OR summer, fall, later, winter, spring?
I believe the reason for not giving a quick "no" to children is so that parents are more open to children's needs and adventures. I'm not sure though, I'll figure it out later.