Too many to just pick one... here are a few examples:
Adding a small stone that is shaped and/or colored in a way I like to my collection.
The look on Arynkitty's face when he wakes up on a good morning.
The smell of rain on a warm day.
Putting a genuine, good natured smile on someone's face who was in a bad mood before.
Watching cuddly little pups at play.
The sensation of being in the right place at the right time.