I am using a stereo 3D pen. What surface should I use? Using a 3D pen I printed a small box. However, I was doing it on plain paper and of course the paper didn't come off the plastic very well. It didn't matter for that specific case, but if I want to print something else, which non-sticky surface would you recommend? Is there any way to use transparent surface (so that I can put a paper with picture as a guide under it)?
You could use a piece of glass, that's what most people using 3D printers have as a build surface.  An easy source of glass for pen use would be a picture frame but the edges are likely sharp so be careful.  Acrylic would also work and is easily obtained in small pieces from places like Lowes/Home Depot, I used Acrylic for some time on my Kossel.  The plastic can stick to Acrylic very well but I had no issues using it with my printer, just test it out and see what process works if you go that route.