I got one of those kits where you take a baby's handprints for my son, but I haven't actually gotten around to framing it. Mainly I just have photos of my kids, which I treasure. And I suppose there are the trees we planted over their placentas (a lime tree for my daughter and an Illawarra Flame Tree for my son), so I think of them when I look at the trees.
Wonderful ideas with the trees. My sister has planted a tree at the birth of her firstborn and she says she has two babies now! :) The tree will grow as the child grows. I think this is a wonderful way of connecting your baby to nature.
I don't have any significant keepsakes, because soon after having the babies, we decided to move overseas to Canada (and then later decided to return) so we had to dispose of quite a few things. I do have photographs and video clips.