You can turn off all data and still use Wi-Fi by enabling airplane mode and then going to the option for Wi-Fi and turning it back on.
It won't help you with your bill, though. AT&T requires a data plan with the iPhone 3G, whether you use it or not. Some users reported that they were able to get AT&T to cancel the data plan for their first generation iPhones. I'm not sure that works any longer from some reports I've read, but the first generation iPhone was not subsidized by AT&T, so they were not recovering their costs by the contract charges for service.
GPS isn't counted as data, but it won't work in airplane mode. Not sure what other apps you are referring to, but things like Google Maps work with Wi-Fi.