In the Safari settings, you can disable syncing the reading list, which is relatively low bandwidth depending on how you use it.
In iCloud > iCloud Drive, swipe to the bottom and you can disable syncing all items in the list over cellular.
In iTunes & App Store, you can choose whether or not to use cellular data for automatic downloads (four different options for things you've purchased and updates), iTunes Match, and iTunes Radio.
You can also enable or disable cellular use via settings within certain apps. For instance, the podcast app Overcast has a cellular data switch in its Downloads area to let you grab a specific episode or download any available episode via cellular whenever it's available.
More generally, you should avoid using or disable the cellular use in Settings for:
Audio-streaming apps, such as those used by radio stations and networks. Usage is generally small, but it can add up.
Video-streaming apps like Hulu Plus, YouTube, Netflix, and Vimeo. It's easy to run through a gigabyte or more in an hour, depending on your device and connection.
Photo-browsing apps like Flickr. Depending on the app, even swiping past a photo might download a megabyte or more.
Your cellular iOS device will warn you if you start running out of data or start to near your current plan limit during a billing cycle.
The Maps app used to consume lots of data because Apple loaded image data from Google to power its software, even after Google switched to offering vector data for plain maps. Vector data uses scale-independent points and arcs and straight lines between them to represent maps, using vastly less data.
Apple's own Maps app and the revised Google Maps app both use vector data. In looking at heavy usage of Google Maps for a three-month period, my iPhone shows only 94MB of data consumed over cellular.
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