My first thought was to laminate activities you bring into the room yourself such as extras or emergency plans. If you bring in your own activities, it is almost certain that the teacher won't be taking them for a grade. If you don't want to make a million copies for each and every classroom you enter, you could make one class set and laminate it. Take it into a classroom and have the students do the activity. You can look at it (or even photograph it if you want a record), and then erase to use again with the next class. I think it would work perfectly for story planning sheets (once the rough draft is written, you don't need the plan anymore) and other graphic organizers that work for multiple grade levels.
Another thought is for fun, time filling, educational activities. One of my favorites is Mad Libs. I enlarged a bunch of Mad Libs on the copier and then laminated them for a student teaching assignment I had several years ago. I store them in a large plastic envelope so they are easy to grab. I have used those things in nearly every classroom I have been in. The students LOVE them, and, little do they know, they are getting grammar and parts of speech practice. Sneaky! Sneaky!
My other idea was to simply laminate plain paper to create your own portable white boards. You could use any color paper you'd like, although I would a light-colored paper so the marker will show up. And you could laminate as many as you wanted... a whole class set, enough for students to work in partners, or enough for students to work in groups of three or four. They are lightweight, easy to store and carry, and provide you with another teaching option even if there are no individual white boards in the room.
Now for all these ideas, you will also need to purchase a set of markers, and that will be a slight investment. Again Amazon has you covered with a set of 12 Expo Low Odor Fine Tip markers, but you can find these at any office supply store and even Wal-Mart and Target. I do recommend using dry erase markers rather than the Vis-a-Vis overhead pens. The dry-erase ones will wipe off easily with just a tissue. The Vis-A-Vis ones also wipe off easily, but require a little liquid to get them to come off, and that just does not sound like a good idea in the hands of students.
If your DIY white boards or other laminated sheets get to where they don't wipe off easily, the Mr. Clean Magic Eraser is just that... magic... and will make them like-new.
And if you are not quite as obsessed as me and don't want to buy your own, your can take your things to an office supply store or teacher supply store for them to laminate for you.
Yea, so you can just call me The Laminator... I'll Be Back (said in my best Arnold voice).