I made four trips to Europe with pre-paid T-Mobile service, meaning no access to T-Mobile's great deal for travelers. I feel your pain. I finally converted to a regular plan this year, and I loved being able to web-surf during long bus and train rides.
During the four previous trips I used a mapping app (CityMaps2GoPro) that allowed me to download a city or country map via Wi-Fi and keep it available on my phone. Not having to check every morning to be sure the map was there was convenient, but the database wasn't robust, and I'm sure there are better options. I never looked to see whether that apo would provide turn-by turn directions. I just followed the You Are Here dot to be sure I was headed in the right direction. I don't rent cars in Europe, so Iblvwas walking rather than driving.
There are other apps that work in a similar way, probably with more complete databases of street names and sights.
Or you can use Google Maps offline or Google's MyMaps, my current preference. Those two require some up-front work by you, again while you have Wi-Fi access. But the initial set-up can be done on a regular computer, which for me is preferable to flagging sightseeing targets on a phone or tablet screen.
In all cases you should be OK once you have used Wi-Fi to download the maps and pinpointed your places of interest, but that was not my actual experience with MyMaps. I frequently had to re-download the map I was using every morning, and it sometmes took a long time (over 10 minutes). I have no idea whether that was due to hotel Wi-Fi shortcomings or issues with the Google server. Sometimes the download hung up for a long time with none of my places shown, and twice I initially had just my colored dots with no underlying map at all. It is because of experiences like that that my first or second stop in each city continues to be the tourist office, to get a paper map.
While your base map and any flagged sights will normally be available as you walk around, you may not be able to use the map's search function to find new places when you aren't in an area with Wi-Fi. I think some mapping tools are better about this than others. An app that resides on your phone like CityMaps2GoPro should provide look-up functions without Wi-Fi access, but it will only be as good as the underlying database (which I found to be rather limited).