You would probably get more answers if you would provide more info, such as where you plan on selling (like city, state or country) and a little bit about the demographics of your target clientele.
One little thing before you start baking... check with whoever has the control over food vendors in this street market.
You did not mention if you already have the proper license as well as insurance (if even needed, I don't know where you are) that most American states require to operate a business of this type.
A business plan is always helpful (for even the smallest of endeavors) it will give you some experience for when you graduate and need to do a "real" one. lol.
Other than cookies, I have noticed that small jars of preserved fruits and veg (jams, jellies pickles...you get the idea) sell like pancakes.
Homemade soap (the old fashioned kind made using lye, not just melted soap from a bag from Hobby Lobby) with herbs and other such additives are also a big hit.
Just depends on what I asked in the first line... the demographics of your target clients.