A few possibilities:
Verizon has a plan called TravelPass that you can add to your phone. It costs $10/day for each day that you use the phone outside the U.S. If you don't use the phone (or only use Wi-Fi) on a given day, no charge for that day. Any data that you use is counted the same as if you were in the U.S.
Google Maps allows you to download the map data for a region into your phone. Once you've done that, you can navigate within that region with no cellular connection.
And maybe it's worth the 50 bucks to get the European maps on your TomTom. I have a Garmin GPS with European maps, and I still bring it along when traveling there, just in case.