Teh longest "day trip" of my life was a couple of years ago. We did the day trip due to extenuating scheduling circumstances.
DD stayed in town (she was in 6th grade) as she had already committed to a "relay for Life team". Then one of my cousins decided to get married that weekend about 5 hours away. We didn't want to be gone overnight on Friday, since DD was at the relay for life. So - we left our house about 6am for a 1:00 wedding, and the reception immediately followed the service. We left the reception about 5. We stopped on the way home to say "hello" to my BIL's family, and then drove home, and got homme by 10 or 11.
This was awful. I will never intentionally have that long of a "driving day" again. It simply wasn't worth the fight DH and I got in on the way home (I was too tired to drive, and DH got pretty irritated at me...blah, blah, blah - he drove all the way there - how could I be too tired to drive home. He was driving in the daytime, and I got the night driving time.) And, looking back, it wasn't worth the "what if" I would have fallen asleep at the wheel and got into an accident.
A good day trip IMO is a couple of hours away - Chicago is a great day trip (but a better long weekend!) and Madison is also a great day trip from the Milwaukee area.