My business is mostly word of mouth. In the beginning, I would make an extra cookie or two for every batch (still do, just in case!) and then wrap them as if they were being sold and just take them with me wherever I went. When I ran into friends, I'd share my "rejects" with them. Also, my licensing requires that I have a full ingredient list plus business contact information (physical address and phone number, I added website) on each package so each cookie that goes out of my business is like a business card, too. Because my cookies are always individually bagged, that also means that every time a child takes one home from a birthday party, or someone receives one as a shower favor, they have my information right at hand.
I'm not a huge fan of spending money on advertising because I think the same money is better spent on producing cookies to hand out to friends. I have done a few donations to school auctions, etc., but only to things I want to support, not really as a way of drumming up business. I think the best ad for my product is to actually experience it.
Since you are already doing cakes, maybe you could have a standard cookie design that you tuck one or two cookies into every cake order free of charge. Or, if you're ambitious (and have the time!) you could make one or two in a theme that coordinates just to show that they could have had party favors or additional desserts in their theme.
Best of luck!