You can use policy manager to log activity:
http://www.cisco.com/univercd/cc/td/doc/product/ismg/policy/ver22/audit_22/audevs.htm#82684
The raw logs from routers wont give you what you need. If you use a PIX, you can use PIX Firewall Manager for detailed reporting or even better Private I from Open Systems (www.opensystems.com).
I think you can generate reports from Cisco Secure as well.
Im sure there is other ways as well so maybe someone else has some suggestions.