If you want a rather long..."day"...
My last trip planned, last year, (which, didn't happen, for reasons I won't get into) I planned a trip over to CERN.
https://visit.cern/tours
My thought was to go for one of the self guided options, though they do run guided tours throughout the day on schedules...I figured I would look into those when I got there, so as to not weigh down my schedule.
You can take the train from DLP to Paris, and then from there to Geneva. Takes about 4 - 5 hours.
You can also find cheap flights that take about an hour, but travelling the countryside by train was part of my plan when I mapped it out, so that was "part of the experience" I was looking for, as well as notching up an extra country (Switzerland), another world city (Geneva) and a unique experience with some educational value in a topic that I find interesting (CERN).
It's a VERY early morning, and a late evening, but you can nap on the train.