Go to B&H and you can do pretty good there with $400.00. :) Better than you would in a store in town. :)
What type of shooting do you do, that would be the first question. But without it, I'd look at an off camera flash, and maybe a bracket (optional I use mine about a third of the time), maybe another lens, used lenses work great until you can afford a new one, and sometimes you never do need it. I have three used lenses and you'd never tell it. cleaning kit if it did not come with one. tripod, and just look for things that would help you in what you are shooting.
I have a portable studio, but that is because of what I do, and most my work is with film, so I use a lot of filters and effects. Depending on what you do, you might need some of that, or other items, or none.