Hi Matt,
Great question. October is the height of the rainy season in CR. You will definitely have rain in Arenal, and potentially quite often. When it’s rainy, the volcano tends to be clouded over if that is something that’s important to you. It will be green and lush though and absolutely beautiful that time of year, so I guess it depends on how adventurous you are. Playas del Coco is in northern Guanacaste, so dries up the earliest, so you will be fine there and will probably see some rain but not for a whole day at a time. You never know though with the weather so it’s always tough! We have a post about the rainy season in CR with some more info about what it’s like in different regions, etc. You can check it out here: https://www.twoweeksincostarica.com/visit-costa-rica-rainy-season/
Not sure if you’re interested in visiting the Caribbean side at all, but Oct. is one of the driest times there so a great time to go. Hope that helps with your decisionmaking!