you should do "from the mixed-up files of basil.e.frankwiler" by e.l. koninsburg. it's appropriate for all ages and it's about this girl, and she runs away, but her brother finds out, and wants to come. the go and live in an art museum, and figure out the mystery of "michelangelo's angel" and end up living w/ basil for a couple days, then she puts the children in her will (she is a very wealthy and elderly woman) and she makes them promise to keep the secret of the angel a secret.