I think scoring people's cakes is more likely to de-motivate people, especially the lowest scoring. And if you think they won't compare scores with each other, I think you're kidding yourself. And then the ones who score highest might be likely to do a wrose job, because they don't want to stand out from the rest. It really depends whether the reward is more valuable than fitting in with their work mates.
Kids love star charts - adults tend to feel like they're being treated like children. Plus, as people have already said above, some won't want to wear an apron, some don't drink coffee, or never get to Starbucks. Certificates are just more paper that I have to deal with. I'm already drowning under bits of paper at home.
I think quiet encouragement of individuals is a better way to motivate. Mention to each what they already do well. Praise their efforts for a while - a week or two? Tell them something about themselves that makes your job easier - could be that they're always on time, or they're fast with some technique, or that they're consistent with something, or that they do particularly neat writing, or can smooth icing especially well.
You don't have to go overboard. You're just trying to establish that the worker is a valuable member of your team. Making them feel like they're helpng you personally by doing things well. I get that finding something might be more difficult for some workers, but find something. If there's nothing they do well, maybe your store has a performance counselling procedure.
Then continue with the praising, and start saying, 'Hey, can I show you how to do X? I love how smooth your icing is - if you just do X,Y and Z, your piping would make the whole thing look better.' And then praise any improvements you see in that worker in that area. Pass on any praise customers give. Praise your workers to your customers within hearing of the worker. "This cake was done by Mary Sue - she's our roses expert - she did a great job on your cake!"
The idea is that people like to know that what they do is valued by someone. That they're helping somehow. Obviously they know that the cakes go to people for their birthdays. But if they're picked up in a box by someone looking for the cheapest cake they can find within 20 miles of their house who doesn't even look at it, the motivation of seeing a customer thrilled with the unique and beautiful cake that custom designers get is lost. So they need to know that you, as their boss, is appreciating what they do.
Long term, that kind of thing works better than a $5 coffee voucher.