For my last trip I found the following Android apps useful*:
Google Calendar – If it’s not on my calendar it not important to me. I used it for adding reminders for the trip. Also I can schedule flights according to the correct timezone for both departure and landing.
Evernote – I keep my packing checklist here. It’s useful to share the list with my travelling companion to make sure we have the stuff we need.
Go City Card – Kept a QR code version of my multi-pass tickets here. It was easy to retrieve and scan at ticket counters.
Google Play Music – For playing music via Bluetooth in the rental car.