-If you need an office suite you have some choices: iWork apps from Apple are free; MS Office for Mac is available, but the current version is from 2011; OpenOffice, Libre Office, etc. are free.
-Browser: You already have Firefox, so there's that. I'm not totally sure, but I'd assume you can download Adobe Flash for Firefox. You would need that if you're using Safari, but Chrome uses it's own built in version.
-Sleeve or some sort of case if you're going to be taking it places
-External hard drive depending on the size of your internal drive; Western Digital or a Time Capsule from Apple are usually the recommended options.