I do "English Passports" and give out hanko points in them for those who volunteer to answer, present in class, or win a game with my SHS 1st and 2nd years and at the end of the year, I do a lottery and give out small prizes from the places I travel to. I'm not saying you should do the same with the prizes but even small things like postcards, pencils, and candy go over well.
I do find that doing the hanko points does get the kids to volunteer more and be more outgoing since they know they'll be rewarded and want to work hard for it.
I know some say that bribing students isn't a good thing and they should want to learn but I do notice that the kids do volunteer more. You'll learn quickly that Japanese students don't like to stick their necks out and for an ALT coming from an academic culture that encourages volunteering to give answers, it might make you really frustrated that students don't give the answer.
I didn't do hanko points when I got to my school in August since a)I didn't know what my predecessor had been doing and b)I didn't think of it because honestly, I didn't think I would need it. But I asked my teachers if we could try it in the new school year for April and in the end it went over well. When I asked my then 1st years if they wanted to do hanko points as 2nd years, almost all said "YES!"(I think seeing the prizes they could have won influenced their answer
Definitely make sure it is O.K. with your JTE's first so maybe don't spend too much (or any money) on prizes yet. Like others have said, look to see if you can free things that represent your country or even your area (I have post cards and key chains from my state in America). Also make sure that it is very well planned out and if you do want to give out prizes but you think the students might not take to it, you could always show them what they could win if they get hanko points.
My own opinion is that the "English Passports" with hanko points makes the lessons the students have with me more special. It's something that is used for my class only and it encourages them to work hard if they are the competitive type. I also feel like it gives the lessons with the ALT a bit more importance if the students have to remember to bring their sheet to get hanko points (if my students loose their English Passports, I tell them they'll get a new one but the points start over at zero or if they forget it, then they don't get points for that day) instead of the class just being "filler" or "just for fun". It helps to put my lessons a bit more on par with their other lessons- if you loose a worksheet or forget your homework, there is consequences- so the same is true for my lessons.