Re: Any Calendar app changes in iOS 10?
sadly no real changes, beyond the widening of the "events found in apps" iOS platform feature and the changes in settings to seperate the calendar/mail/contacts components (instead of one sub menu).
I can't believe they haven't added free form event entry, they've done it on the stock OS X Calendar app.
No big deal for me, as I actually enjoy using Calendars 5 by Readdle (it also has a great reminders capability, with a few minor irritations, it is better than the iOS reminders app), they need to update with split-view support for iPad. I also tinker with Fantastical from time to time.
I've not checked Google Calendar app for a while, my last quick check was that it looked great, but I couldn't get a feeling for the spacing of my day (ie interval between meetings etc).