I remember kind of freaking out about walk-ins before the fact, because, let's face it, if you're on here you're at least a planner and possibly a control freak (I was), and these things are hard to plan and/or control, but as @sopranomtmom says, it's pretty self-explanatory once you get there. You walk around, you ask people and you bring a checkbook. My S did one and got an offer from it and thought about another one but ended up bagging it.
My son did an internship with Oak Park Festival Theater in Chicago suburbs between freshman and sophomore year. There’s a very small stipend (he worked at a donut shop for money mostly) and though all the interns u/s and have small onstage parts, performance opps vary a lot from year to year. He also got some equity points but this is also not a guarantee (you have to have three equity people in your show to get them and sometimes there are only two). It’s worth considering if your kid has a place to live for the summer in Chicago.
Excellent post, I would also adddo not freak out if you don’t get an agent right out of showcase. Sure it makes things easier, but you can do a lot with self-submissions and small projects can turn into big ones if you knock it out of the park.