Here is what worked on long trips for us:
If you are taking snacks (we liked non-sugary, so they wouldn't get hyper) and your baby has the motor skills, take it in a container that is a little difficult to get the snacks out of. (We have a Cheerio container that looks sort of like a Cheerio, but only a part of the lid lifts up.) It cuts down on the mess, but it also keeps them occupied longer.
We did sort of the same thing with a few toys. We had a draw-string bag that was open enough to get the toys out of, but took a little manipulating. My boys just like the challenge of taking the things out.
Some big beads or buttons on a string works
sometimes. I would make it with extra string (my boys wouldn't like beads behind their necks, and liked moving the beads around), and the child can "wear" it. They can twist them, move them on the string, eat them .... This can also go into that drawstring bag

.
My friend usually gets a few toy (for a long trip) that are new to the child and keeps them hidden, then brings one out every now and then.
My other suggestion would be to travel through the night if you can. It is great when they can sleep through most of the trip. If not, and if there are other children on the bus, it is good if the child can see the children. My boys were often entertained just watching other children be children.
Other than that, try to stay calm and cheerful. Babies seem to pick up on our moods so easily. If we're having a great time, they often figure they should be too.