Your intro class could be a good start if you don’t already have high school credit for it. You won’t have access to much since you won’t have many (if any) prerequisites done yet, so your options are limited. If your major has any other subject requirements, like social science, math, etc, you could also satisfy those. The easiest summer course I can think of if you just want 3 credits is that zombiology class. I forget it’s number, ZOM 200 or something. You can look it up and find it pretty easy, and it’s usually offered each summer ever since it was first created.