There were several vacations we took growing up that weren't planned. I had no idea until I got older that many families spend months working out details of their vacations. They have the hotel booked months in advance, decided on which attractions they are going to see and when.
Plan a vacation of where you are going, load up the car and head that way. (Road trips can be made to be really fun).
One time we arrived in Destin, Florida and my dad drove down the strip and let us girls (who were young) pick out the hotel/condo we liked the best. He would then go inside and see if they had any ocean front rooms available (a must for my dad). Some places were booked but we always found the perfect place and we would get so excited when he would come back and say, "Saddle up! We have a room girls!"
We had so much with the "adventure" of last minute planning and never knowing what was next. We also were asked by our parents to request special breakfasts made by Mom and at least one trip to the cheesy souvenir shops to buy beach toys that were almost always broken by the end of the trip.
The stress of planning a trip can sometimes take the fun out of it. Try not planning as much and let yourself (or your kids) make last minute decisions. They can make the sweetest memories.