Is there a system or secret to collecting the souvenirs? Is there a bonus for getting all of them? Seems like I collected them randomly and fairly quickly at first. Got to where I have all but one but that was long ago and I never get the last one. I thought it would pop up when I got through the 100th trip but that did not happen.
They may never give it to you in hopes you keep playing it! I somehow have 10 souvenirs or so, but they never mentioned I received one after my very first. Odd.