How to remove internet usability traces in Ubuntu?

I've rented a laptop from my college and need to give it back soon. I haven't done anything illegal or bad with that computer, but still I want to give it back as cleanly as possible. I mostly want to delete all local internet usability traces. Since I'm new to Ubuntu, I'm not sure if there are some logfiles in the background somewhere (for example logging all url's visited and so on...). You Linux veterans, what files to delete and where to find them? Thanks!