Open the back of the phone up and take a look at the SIM card. There should be a logo on it which will tell you what carrier it belongs to. From there, you can look up the number of the carrier, give them a call and let them know you found the phone (they'll ask for the IMEI #) and how their customer can contact you to claim it.
This way, the phone's owner can contact you and offer you a reward for finding it so they can clear out the contacts and use it on their new phone.
On the flip side, the owner might be upset that you found it, as that may skew his/her insurance claim on the phone!