That sounds like a good plan. I would at least start looking now into tours you can do while staying at Andaz Papagayo. While it is physically not far from LIR airport, your transportation options from the hotel are limited if you don't have your own vehicle.
An alternative for your last two days could be:
Saturday - stuff near Arenal and transfer to SJO airport hotel
Sunday - fly back to USA from SJO
... depending on where you decide to fly out of.