fostering musical talent?

My 4 year old really loves music - loves to listen to it, and loves to sing and play her toy musical instruments. She has good pitch and rhythm. I'm not musical (well, I never developed any musical talent anyway), but my brother plays guitar and is very musical, and he is impressed with his niece's musical aptitude. So, my question is, what are some things I can do to help foster her musical talent? I don't want anything too organized at her age to make it a job and not fun, just some ideas beyond what's just around at our home that, in an unstructured way, will provide ways for her to develop and enjoy her musical talent. And, at what age do you think any formal lessons would be appropriate. (I KNOW she'd hate that now.) For example, what age do most kids who play piano start lessons?