Take a picture on their first day of the school year, frame it and then take a picture on their last day of the school year holding the first picture ... I also love the idea of graduation day holding the first day of kindergarten!
Write in kids' quotes with dry erase marker, then take their picture underneath. Put them in photo albums to look through later and see the funny things they said through the years. For example, "We've been waiting here so long I forgot Josie had braces!"