My children are 4 and 3 and travel 3 hours at least once a month to visit dh parents. We try to leave after lunch and they normally fall alseep within an hour of the trip. Sometimes not and then we chat or see who has cows, sheep, or horses on their side. They also have some books and their toys with them.
I keep a small cooler on the floor behind my seat and have water , pop or juice in bottles too. Some small snacks as well. With my dd being 3, we take her potty with us (she doesn't use it at home as its the travelling one). That way if we have passed a town or gas station and she says I have to go we can pull over onto a side road.
DH and I use to travel late at night too before kids. It really is a great way to go - no traffic and no cops out at that time of night. I used to manage a fast food restaurant and would get home anywhere from 1-3 in the morning - so we would head out then.
We did take ds to Montreal one year to visit a great aunt and travelled at night. He slept all the way and when we got there he was awake and we were all tired.

And guess who got to stay up with him! Little sleep does not do me well and I missed going to see the Canadiens Play as I was dead tired
Hope some of these tips help.