Firs things first, back up your device before tomorrow. Then, go through and make sure any third-party apps that use their own syncing service are synced up too. A lot of apps have stopped using iCloud for syncing, including Day One, LastPass, Dashlane, Simplenote, and plenty of others. So, if you haven’t used an app in a while, pop in and make sure it’s synced up properly.
Backing up and syncing is generally just a good idea, but it’s also important because iOS 9 introduced a new feature that temporarily deletes apps to free up space for installing updates. This sounds awesome on the surface, but carries the risk of deleting data from apps that haven’t synced properly. As always, it’s better to be safe than sorry here, so make sure everything’s up to date.