You could say "Everything is done for the day." Some other possibilities, tho these might change a bit from your original, could be: "Today's work is all done", "All of today's work is done". Also, you should say "he can come by my office at any time" instead, "he can stop by my office at any time" or "he can drop by my office at any time". Hope this helps.

Today's work is done. Today's work is finished. Today's work has been completed. I've finished all I have to do today.

You might also say "I've finished all my work for today." Another way you might hear is "I'm all done with my work for today." All of Robbies examples of telling him he can come to your office will work.