We have made this trip every year since our daughter was born. We actualy went twice the summer she was born. Everytime we have gone we have only stopped once in the 5 hour trip. We stop at a McDonalds in Shaboygan and keep on going. We plan to leave early in the morning when she is happiest. We make sure to pack a portable DVD player and lots of things to play with in the car. We were just up there over the 4th and LOVED it once again. My daughter is 4 and on the way there she played with her dolls and watched two movies. On the way home she slept 1/2 the time and played the other 1/2. In fact we drove straight through to get home. One thing that helped us on our trip (this one and one we took in june to california...4hr flight!) was to have a few hidden new things to do in the car. When she was getting a bit restless we surprised her with something new. They were little things like a color wonder book and markers, etch-a-sketch, new doll accessories for her bitty baby, a new vreader book for her vreader, a new dvd...etc. My opinion has always been not to prolong the trip. Getting in and out of the car tends to make my daughter grumpy and the "are we there yet" starts coming out.
All that said, enjoy the beautiful weather! We absolutely love Door County and so does our little one.