My three grown children can still recall when we used to take surprise day trips in the car.  I wouldn't tell them where we were going but, I would tell them that once they got into their carseats we would be starting the surprise trip.  They tended to react quickly and get into their seats without additional fuss.  Once in their car seats I would either tell them to close their eyes or I would use some of my husband's old ties to blindfold them.  After doing this many times they would tend to be more quiet on the way to the location so they could try to figure out where we were going.  So whether it was to the mall, Chucky Cheese or to the park they were usually very excited and much more quiet along the way.  Best of all now they recall these times very fondly.