Without any additional apps, iOS devices can be a pretty powerful device. Here is a list of how you can use most of the apps that come with the iPad right out of the box in your classroom.
Maps: I teach social studies so geography is always something to teach. Although Google Maps has a great database of historical maps, Apple Maps works just fine for looking up states and capitals, the location of current events around the world, and much more.
Notepad: We use Pages in my classroom for most of our work, but sometimes I don't want students to create an elaborate document or even turn in what they are doing in class. Notepad works great as scratch paper for students to write down quick notes they only need during the class period.
Speech to text: Many students don't like to type, especially on an iPad. iOS has great speech recognition built in. Students can hit the microphone icon and start talking, iOS does the rest.
Camera: A picture says a thousand words, or so the saying goes. Having students use the camera to create videos or use pictures to show their learning requires abstract thinking and creativity. For many students it is their preferred form of showing their learning.
Siri: Students always find questions I don't know the answer to. Now the response is easy... "I don't know, Ask Siri".
Airdrop: Now that airdrop supports Mac computers as well as iOS devices sharing documents is super easy between students or student and teacher.
Restrictions: In the settings app under "General" you will find "Restrictions". This handy resource allows you to turn on and off features like the App Store, in App purchases, game center and most importantly lock down the account used. This makes it so that students cannot change the iTunes account to their personal account and download games or other apps onto the device.
Podcasts: There is a lot of information available through audio and video podcasts. One of my favorites is the CNN student news video podcast. A quick run down of the days news meant for kids with a great host. Highly recommended.
Accessibility: Again in the settings app is the accessibility settings. These really come in handy for students with disabilities. Whether it is making the text larger, converting text to speech, increasing contrast, there is a lot there to help students out. It also allows for guided access which locks the iPad into a chosen app. This comes in handy for students who are easily distracted by other apps available on the iPad.
I'm sure I missed some. If you have some unique ways to use the apps in iOS that I didn't include please let me know.