Brian,

That is awesome. That is a part of Costa Rica I haven't been too, but we have looked into it. Before thinking of the gear you are going to take, you should consider your plans. There is likely some estuary/river wildlife cruises in the area. Also, not far are the Santa Rosa National Park, which is supposed to be great for birds and monkeys, as well as the Ricon volcano, which is famous for a "blue" waterfall/lagoon.

But there are also a lot of resorts and all-inclusives in that area, so it all depends on what you'll be doing. But my basic kit on this last trip was the 24-105L, 100-400L and 100L macro. And I only used the macro for the frogs. You likely could get by with a basic kit of the 24-105 and your 70-300L. I'd be tempted to get a graduated ND filter if you intend to photograph sunsets, which you'll be in great position to do.

Feel free to PM me.

Glad to hear about your trip.
Brant