A good camera and a camcorder... and Photoshop as well as video editing software. I have a 7 year old son (from before I met my husband), and it blows my mind how quickly the years go by. You want to be able to capture all those moments... before they've changed and grown up even more.
Also, having a good jogging stroller has been wonderful! We took lots of long walks together when my son was young.