It's so hard to make all the songs go together and you really have to start with a style that you like and just keep listening until you find songs you love that go together well (assuming you need more than one song.) I'd go through the music you play for her - if there's a special lullabye you always play, try to use that as a starting point for the style you want. There have also been some great suggestions already - I'm bookmarking this for reference later!
I have had good luck with searching for classical guitar and flamenco songs. There's a lot of stuff available and it all kind of goes together. One of my favorite instrumental finds is Treasure Bay by Govi (from Guitar Greats, Best of New Flamenco.) Upbeat and moving at the same time, and makes me think of summer.
There's also some great fun stuff on children's albums - I'd look at Ralph Covert for rock-n-roll. We've used one of his songs called "All I Wanna Do is Play" a lot for kids slideshows. Celia Cruz has some great stuff too (Spanish.)
Another perfect song for pictures of kids and family having fun on summer vacation: "Wastin' my Time" by Eagles of Death Metal. (Ignore the band name, I think it's ironic - not a death metal song ;-)
For a kind of sad, sweet mood: Let it Shine by Randy Newman (from Sail Away). (I don't know if the lyrics are exactly right for a baby, but it's a great song.)
One of the happiest-sounding pop songs ever: Light and Day by Polyphonic Spree (I didn't realize this was from the Eternal Sunshine of the Spotless Mind soundtrack!) "just follow the day and reach for the sun!"
If you have cute pictures of babies hamming it up for the camera and are looking for a light, funny song to break it up: Supermodel by Jill Sobule (from the Clueless soundtrack)