Express pass is a must....I'm sorry to say. I have gone to the parks with and without. Having had the taste of EP, I would never consider going without. But the EP lines can be somewhat long sometimes depending on the ride, especially Minion Mayhem! they definitely aren't walk on, but similar wait times like the Fast Pass lines at Disney. We are definitely able to do more. Last year at Spring break we went to the parks one day without EP as we only stayed onsite for 2 nights but had four day tickets. Our day without EP, we barely did anything. We just didn't have the patience to wait, and we knew we were coming back for three more days with EP.
The line up to get on the Hogwarts Express was super long every day we were there...they didn't have EP last year either. And one day, the ride was down almost all day. We actually had to walk out of the park and walk all the way around to enter the other park instead of taking the Hogwarts Express.