Rock 'N Play, the Rainforest Bouncer seat, and of course the My Little Lamb Cradle Swing.
As my baby grew, we had to retire those seats ... but the lamb swing continued to be a permanent fixture next to the couch. She hadn't used it in months, but for some reason, it took us a little too long to take it down and pack it up. Although it's enormous, and gets in the way of everything, taking it down was a reminder that she is growing up way too fast. Everyone always says the days are long, but the years are short. And I'm starting to believe it.
What was hard for you to part with? (Those tiny newborn onesies are a close 2nd.)