Excessive internet usage

Hi all,

I'm experiencing excessive data usage, as measured by my ISP. I'm talking some 5 Gb per day, without downloading or upgrading software, watching online movies, etc. I've got an iMac and a number of portable devices in use by the family (iPhones, iPads, etc) but I can't figure out what may be using all of this data.

I've got an Airport Extreme to connect to the Internet, with (>10 characters) WPA2 password protection and even exclusive access to the router on the basis of each device's MAC address, and I have no guest network. I can't therefore imagine that someone else is using my bandwidth. Yet, I also can't imagine that it's me. The MAC's firewall is on, I am not using iCloud (other than for notes and contacts only), so there is (should be?) no synchronisation going on for documents, pictures, etc. Could anyone please help me to find out what may cause this excessive data traffic?

I've seen some posts about some of the tools that may be able to monitor data traffic (Little Snitch, SurplusMeter, MacProxy, and others), but I'm not sure which one is to be trusted . . . . and which one I would be able to install, setup and use, given that I am not IT savvy. Is there any tool 'built in' to the Airport Extreme, that I can use to identify the culprit? Is there any way in which I can even verify that the numbers advised by my ISP are correct?

I'd much appreciate your help, as I am forced to upgrade my ISP package time and again, due to Internet usage that I don't think is mine . . .

Thanks and advance for your insights and assistance, Peter