To propose at her work can be really thrilling and fun. It is important to take into consideration her kind of work and the type of setting she is in. You have to get permission from her boss and supervisors that you can pull this off in the workplace. The last thing we want is her getting fired because of your proposal!
The exciting part is that the last place she is expecting a proposal is at her work place. Depending on her work, you can surprise her in different ways. If she is in an office setting, you can have her boss call her into his office. You will make prior arrangements with her boss to have this done when he is actually out of the office and you can pretend to be her "boss" sitting in his chair.
At that point, you can have others come in the room to surprise her or keep it more private and propose there. This idea can definitely be followed with something romantic and fancy like a night out salsa dancing, candle light dinner, or a fancy restaurant.