Hello, Bjorn!
Even with a
one park per day theme park ticket, you can leave the park you start in, go back to the
resort or to do some shopping or dining at
Disney Springs, and then when you're rested, go back to the same park you left earlier. As long as you keep to one park per day, you can come and go from that theme park as often as you'd like - or as often as you need to - over the course of the day.
I find a great plan of attack for my kids is to hit the park early in the morning, and let them see and do all they can until lunch time. Then we'll eat either in the park or back at our resort. I can make my
FastPass+ attraction selections for when we come back. Even if the park has gotten busier over the time we've been gone, I know we'll have at least three attractions we can do by using the FastPass+ lines. It has always worked really well for us.
Wishing you an absolutely magical vacation!
Anna