Since my ds wakes up every 1-2 hours at night, even in the car, we have actually found it easier now to drive during the day. When he wakes up from a nap, he is usually pretty content for a little while until we find a good place to pull over. When he wakes up during the night, he cries until I get back there and lean over to get a nipple in his mouth, and then I sit there with my neck kinked for 20 minutes. If he didn't wake up once asleep, I would drive at night.
As for places to stop, we have found quite a few areas that we keep an eye out for, since it is usually pretty hard to find a park or playground when you're on the freeway in an unfamiliar area:
Indoor malls - a relatively safe place to run around, and many have little play areas. Ds loves to ride the escalator up and down, up and down, up and down.
Fast food strips - even though we don't eat fast food, many of the places like Burger King and McDonalds have play structures.
College Campuses - any jr. college or university along the way will usually have good lawn areas to run around.
A big store like Target, Home Depot, etc., can be good too.
We have also sometimes just found a relatively deserted exit, pulled over, and played ball outside the car, and let him sit up front and play for awhile before getting back in his carseat.
As far as keeping him entertained while in the carseat, here are a few things we have done:
Electronic toys. We found one that just has a bunch of buttons and knobs that make different sounds that we keep in the car.
Empty water bottles - I will often put things in them for him to try to get out, and then put back in himself. The last car trip we did pretzel sticks and pine nuts.
Snacks, snacks and more snacks.
Envelopes full of folded up paper (junk mail is good for this). Ds loves to pull it out, unfold it, and check it out.
Playing peekaboo with him.
That's all I can think of for now. If I think of more I will post again.