In the short run, you're probably looking at advertising. Eventually, website optimization might help you gain traffic from the search engines, but that is a long-term tactic that could take months (or even years) to have an effect.
If you want to get quick responses, look in to online advertising, specifically pay-per-click. You can choose to limit your advertising "reach" to only those areas where you have physical stores, if you want. Advertising will get you instant exposure -- as soon as your ad campaign is approved, your ads will be up and running and (potentially) being seen by prospective customers.
Since you have physical locations, local search could also be a good option for you. Make sure you've set up your business profile pages in Google and Bing and that you've created profiles on local service pages such as Angie's List and Yelp.
You may also want to contact other compatible local businesses to see if you can trade advertising or promotion with them. Photographers, videographers, party venues, event planners, party supply shops -- think of the other services that people who buy your cakes might be looking for, and see if you can work out some sort of co-op deal with those service providers. It could be as simple as adding a page of "recommended service providers" with a link and a small blurb for each service provider, or placing small banner ads on selected pages on each other's websites.
(Don't let the "reciprocal linking" Chicken Littles scare you away from this sort of thing. As long as there's a legitimate reason for you link to the other website -- which, since you provide complimentary services that each other's customers might be interested in, there is -- and both you and the other business code the links as "nofollow," the search engines will not care
. Seriously. The thing is, you're not seeking out these links for any sort of "SEO link juice" -- that's why you make sure they're coded as "nofollow" -- you're looking for the actual human visitor traffic
you might be able to get from them.)