Honeymoon Car Pranks

Balloons
Obtain access to their getaway car. Fill with balloons. A few extra helium balloons in the trunk with their luggage is a nice touch.

Add some peanuts
If you can get access to their luggage after they have packed, add styrofoam peanuts in whatever nooks and crannies are left.

Impossible to drive away
Jack up the car, put blocks under the axle, then lower the car onto the blocks. When the newlyweds try to make their getaway, watch them rev...and rev...and rev.