Most significant bus trip and take some time so it is best to prepare with a little food and water. I typically bring a small bottle of water in my backpack to make sure I don’t get dehydrated and I refill it when ever I am need a water source. One of the biggest underrated assets of living in Costa Rica is the water is drinkable.
The second part of the equation is food. The bus stations all have food but it might not be what you want or need. It appears to me the food in the station is “heavy” meaning it is fried and probably would not travel well. I try to 1) eat light while traveling on the bus to make sure I don’t encounter something that will upset my stomach and 2) I am never 100% about food I randomly encounter and it’s quality, ingredients, etc. Remember there are no bathrooms on the bus like you might have seen on a Greyhound in the States. I carry a “barf bag” from an airline trip but there is no easy cure for an upset stomach.
So, one of the items we have been experimenting with are Rambutan which are quite ugly on the outside but sweet and juicy on the inside. The thick outer skin makes great protection while traveling. Unfortunately, there is not a lot of “meat” in the fruit so it is a nice snack but wound serve as dinner. An apple, would be much better travel food but they have to be imported and are expensive.
I think a combination of fruit and nuts might be the most condensed food selection to satisfy your hunger in a small, easy to keep and transport package. Unfortunately, nuts are expensive in Costa Rica but we use these packages to hold us over until we get to a place that serves the kind of food we prefer (more vegetables and less frying).