Deanna's Japan Postcard Puzzle

In the summer of 2013, I went back to Japan for 2 months. Right before leaving, my new roomate Debbie and I had received a whole ton of postcards from our friend Sarah, who was travelling around Europe. They had stickers in weird places and total nonsensical stories on them, and we were POSITIVE that the postcards were a puzzle, although we simply couldn't figure out the code.

So as "revenge" for Sarah's "Pirate friend" postcards, I wrote my own puzzle and sent it to friends signed as "your ninja friend". Sarah's postcards turned out to not be a puzzle at all, but mine actually were... so in the end she inspired something great and I had a lot of fun writing this up and sending it out! Amusingly, though, one friend I sent it to didn't even realize it was a puzzle. (Didn't he think it was strange to receive 6 postcards from me with weird stories on them and a bunch of stickers?)

The people who got them in the mail simply got them spaced out every 2-3 days. As far as I know they all made it there with stickers intact, but maybe not, I dunno. Here is the full set. See if you can solve it! (If you want to submit an answer or ask for a hint, my name is Deanna Rubin and I have a gmail account with a dot between my first and last names.)

I will say that the address part of the postcard is completely not part of the puzzle, hence why it got blocked out, and the USA doesn't matter either, nor do the stamps or the postmarks, nor anything in the "ninja friend" signature, since I had a feeling that'd get stamped over.

Postcard 1

Postcard 2

Postcard 3

Postcard 4

Postcard 5

Postcard 6