go for it. what a fun spontaneous adventure. i would think it would end up a trip you'd always remember just for how you decided to go.
we have a similar trip planned. about a month ago my granddaughter said, "take me to disney and lets sleep over" so i just said ok, where do you want to stay? she's 7 and about had a heart attack but we booked then and there for a trip starting aug 14th. had to do it a little bit out due to work but just the act of booking right then and there has made her super excited. i let her pick where we're having dinner and the resort we're staying at so she's telling everyone she's planned everything. now she's studying park maps and reading a book about hidden mickey's so "she's ready".
so get in your car and GO!!!