Set up an email account for your kid(s). My personal fave is Gmail since it has a lot of space, and it's free!
Once you have it set up, start to email photos/videos with stories to the account to create a repository of memories and special moments. Eventually, when they are older, your child(ren) will take over over the account and have an amazing time capsule of their lives growing up with you, their friends, and their family.
You can even record yourself on video explaining the story which they will absolutely love to look back on when they're older!