Hi, first off, it will be a great experience.
What to expect? That depends from person to person... one person will expect this and the other will expect that from you.
Remember, its gonna be what you want it to be. If you just sit in the corner and do nothing, then thats what you are going to walk away with... a night/day of being bored. But if you get out and ask questions and ask to help with pt care then you will walk away with an awsome experience.
I would recommend asking the medics/emts what they expect from you and what they are will to let you do pt care wise. Then you will not be wondering the entire night, "what should i be doing".
What did I get to do? (I am sure you are talking about EMT-Basic ride along, as you said "need to do one...")
I was able to do a lot of the pt care. The EMTs I rode with would first tell the pt that I was a student and then they would ask permission if they minded me doing care. So yup, I gave it my best shot, and when I messed up the EMT would just step in and take over until I got back on track.
It's hard to say what they are going to let you do and everything.... so just walk in there with a good attitude and lots of questions!