First time traveling by airplane with 4 month old. What are your tips?

We are going to be visiting grandparents in a couple of weeks and it will be our first airplane adventure. It's a 3-4 hour ish direct flight. Baby will be on our lap. Wasn't planning a stroller since we don't have a small one and there is a car seat at grandparents place.

Give me all of your travel tips, advice, hacks, etc.