Generally speaking, gifts to the boss aren't a good idea. Other employees may then suddenly feel they too should be giving the boss a gift and you might be looked on as opening a can of worms and making the others feel pressed to do what you did. That's not your intention but at work you have to think about how others will perceive things, and yes, everyone in the office will know that you gave him a gift.
You mention that you only started working with this firm this past year. You may not be aware if they have a company culture of "no gifts to your superiors." I would err on the side of caution and give the boss a very nice thank you card with a handwritten and sincere note inside, talking about how you value and enjoy working with the company.
The absolute most I'd do for a boss would be something like someone else mentioned -- a photo you took at a company party or conference, with not just boss and you but other employees in it. Frame it (simply) and give to the boss. That's it. If you give gifts at work give the same to everyone. My former boss gave every employee a holiday ornament, all in the same style but each slightly different. I have done small food baskets for each person. Food is pretty much always the go-to because folks can eat it or not, give it to others who like it better, and if everyone gets the same it's fine, but since you say you cannot do food (not even a fruit basket you get for the entire office to share, and put out in a kitchen or common area?), do nothing for the boss alone. If you give a small LED flashlight, give one to everyone you work with day to day and include some fun message like "You brightened up my year at work" or something cheesy and friendly like that, but not just one for the boss.
The size of the bonus has nothing at all to do with a gift for the boss. I know you only mean to show appreciation, but would you do the same if there were no bonus at all? I figure you'd still send a great card and note-- right? Giving a gift after receiving a large bonus could be misinterpreted by coworkers. And I would absolutely avoid any gift cards or "useful" gifts. Stick to something very professional like a photo for his office IF you feel you must give something but overall I would say, no gift.