Do you have unlimited data (or a lot) with Verizon at home? If so, the $10/day plan uses your regular plan's allotment of data (and minutes), so you won't face any penalty for a data overage - just $10. And Verizon charges you only for the days you use the plan. You can sign up for it before you trip and never use it and not get charged anything - or use it one day in case you can't find a SIM in a timely way at the airport or something.
(There's also a Verizon monthly roaming plan with very little data - and there would be overage costs if you go over on that, but not the daily plan, in my understanding.)
I used Google Maps constantly last year on my 17 day trip to Europe, for walking and public transit directions in various cities, and I still used only 2GB of data, a lot of which was for tethering to my laptop on train rides. So the data use for navigation may not be as much as you fear.
Install Google Hangouts on your phone before you leave, so if you buy a SIM you can make free calls home to US numbers (e.g. check your Verizon voicemail for free once in a while). Add a +1 to the front of US numbers to call them from Europe.