I'm not sure if there is self laundry at any of the hotels, but the laundry at the second hotel Arenal Manoa was very inexpensive. I want to say it averaged $2-$3 per piece. Don't wait until the Marriott at the end because it was more than twice as expensive.
We were there in July and it was extremely humid, so I'd suggest packing extra underwear and socks in case you wear more than one per day, and mostly dri-fit clothing. Multiple bathing suits are good, too. Overall people dressed very casual although some women wore dresses to a couple of dinners. Pack a raincoat for everyone and also some type of sweatshirt or light sweater to wear on the bus or at meals where the A/C is on.
Any other questions, feel free to ask!