It depends on where you are, and what you need to do. Variety helps, so you don't do the same thing at every stop. I find shorter but more frequent stops are easier to handle. Easier on the battery too; and reduces problems if you encounter a charging station that you can't use for some reason. That also makes it easier to leave when you are done doing something, rather than looking to fill time until the battery is full.
Some things I have done while charging:
Eat (I start driving when I get up in the morning, and only have meals while charging)
Get some exercise and/or take a shower (one of the few good things about charging at campgrounds--they often have hiking trails and showers)
Sleep
Visit with nearby friends, relatives or EV advocates
Shop, check out the town, visit a museum. Look for things to do next time!
Catch up on my email or reading
Answer questions about the car and charging
Look for birds (my Dad's hobby)
Plan out the next charging stop (I usually plan before I start the trip, but I have gone off-course a couple of times)
Add information about the charging site to online databases
Do all the "trip paperwork" that oftens happens afterwards--file receipts, save photos or notes, put business cards in Contacts list, etc.
Attend a wedding (the one I attended was a planned part of the trip, but I suppose you could just check out nearby churches. There's usually cake)
If it's a long stop, I suppose you could try these although I haven't yet:
Do laundry
Watch a movie