My bakery (home, licensed) does more than just cakes, so I take other baked goods to the farmer's market with pretty good success. I take cinnamon rolls, several kinds of cookies, scotcheroos, quick breads, wheat bread, whole grain muffins, etc., during the summer.
I've "subbed" in the indoor market and brought some of the same stuff but also some mini cakes as well. Valentine weekend, I sold all my cakes plus I could have sold at least 8-10 more, based on the number of people who saw the cakes and asked if I had any more like that. I think the holiday had something to do with that, so I am planning on going the weekend before Easter with some dummies and having people pre-order for Easter weekend. During the summer, it just gets too hot to have cakes outside, so I haven't done that.
You'll have your "regulars" at the market that are only there to buy fresh veggies and then you'll develop your regulars who come to you each week for some goodies and then you'll have people who will occasionally stop by. I'm just always friendly and try to have something to say as people walk by.
One thing you have to consider is that "time is money" and there can be a lot of sitting around at the market to make a little money. This summer, we are only going to go 2 times a month instead of every Saturday and encourage people to place orders for the other two weekends. I'll gladly meet them at the market with an order but I don't have time to sit around for 5 hours on Saturdays this summer. Good luck!