In some ways I wish I knew. I do keep videos on it for traveling.
BUT I'm also seeing that the apps themselves store data but it's all opaque. The yellow spaced used bar in iTunes is getting huge. With no real way to see which apps are hogging all the space and which items are the largest offenders.
All the app sizes on list by size seem to be only the app itself, not the content it's storing.
Also, my photos collection is 160GB, music, 87GB alone. There's no way I can keep even a reasonable fraction on the device. I could use the cloud but with overage fees and throttling that's not really an option now either.