It'll be October, the lowest of the low season. Tours and transfers won't sell out, but you should arrange things at least the day before.
My reason for mentioning making plans before you get to Andaz is that you're a bit isolated there and the things you'd like to do are quite spread out. You should map out a plan rather than arrive there and say, OK, what are we going to do? You'd need to make arrangements with the front desk at least the day before.
Things are a lot closer in the Arenal area. It's easier to be more "spur of the moment" there, although with just two full days, you'd also want to have some advance ideas.
Although the distances are approximately the same, it's easier to get from Arenal to SJO than to LIR.