An honest thank you!
In Japan, it may not be quite as common for strangers to help each other for no reason, but as a traveler, you will find that people in general are willing to help you with this or that, particularly if you express to them that you need help with something. Just asking someone for directions may lead to them personally guiding you there, or even riding the train with you to make sure you don’t get lost. In these cases when someone goes well out of their way, or does something in general good spirits, there is a better phrase than “arigatou gozaimasu”…. “Tasukarimashita” is a more colloquial option, and literally means, “you saved me”, but in more casual situations just means, “you really helped me out”, or “I couldn’t have done it without you”. This is a very common expression of gratitude among friends and strangers alike, and using it properly will really be a pleasant surprise to some Japanese people. Although alternatively thanking them in English is an option.