Firefox is installed by default, I found it was faster / more responsive than Chromium.
Re:office "Libreoffice is a full-featured office productivity suite that provides a near drop-in replacement for microsoft(r) office."

By the way, if what you want is to play with Linux, I would suggest you do a Windows Install, instead of installing to USB since it will run much faster. A Windows Install (one of the options if you mount the Mint Image using something like Daemon Tools) is that it installs Linux as a program in windows, creating a virtual partition of sorts that doesn't mess up your hard disk, then when you re-boot, you finish the installation into this virtual partition and you are set. I've done this and it is great.

The advantage of installing it to USB for security purposes, is that you can stash your bitcoins in the USB linux and throw it in a safer, bury it, or whatever, hehe.