Puppies. I know puppies are naturally cute, but it makes me so happy when they do the "squinty-eyes" face. You know, when you pet them in just the right spot or when they're sleepy and they close their eyes. It's the most adorable thing in the world.
The smell of books. I'm not sure why. There is something satisfying about it.
The sound of water also makes me happy. Either splashing in it, stepping through it, whatever. I like it.
I probably have more, but I just can't think of them right now.
Edit: Actually just thought of this while I pressed the icon to post. For whatever reason, I absolutely love the sound of old keyboards when they click-clack when you type. I love it so much. Any keyboard sound will do, but the "clackier," the better.
Edit Again: Random quotes also make me happy. Callandor's post below me (your quote in the signature) reminded me. Hahaha.
I have a few from video games and a lot from books that make me happy. I actually have a quote book full of them. Some are funny and others are more inspirational. One of them that makes me really happy is from a book called "Just Listen" by Sarah Dessen. A character says, "Silence is so freaking loud." For whatever reason, I smile whenever I think of it.