I would be very careful about actually showing up at his work place un-announced. He may be in a meeting or having an un-pleasant phone conversation with a client, or a vendor may have shown up that he needs to have an involved or time-consuming discussion with. He may not have time to enjoy your surprise and then you’ll end up feeling bad. The idea of a surprise visit might sound romantic, but some offices and workplaces would be disrupted by your presence and your boyfriend might be a little bit embarrased, if he isn’t expecting you.
But the idea of having something delicious sent to the office, like a really fancy cake from one of those frou-frou bakeries, something that he could share with his co-workers if and when they get a chance to eat some, would probably be highly appreciated. Or maybe a platter of sandwiches and cookies from a local deli.