A few hours after night has set in, find a safe bathing spot (check with locals as Costa Rica has dangerous currents - perhaps a local could come with you) and go for a night time dip in the sea.
You will witness a show of light from inside the water. Millions of little lights appear when you disturb the water by moving your arm in it.
These are small unicellulars, that light up when disturbed. They only live in the purest of water, whicj is why they are found on the shores of the Osa Penninsula.
Fondest memory: The stars above and the lights in the water all around... It was defenatelly one of the strongest moments in my life.. incredible, beautiful, peaceful... a communion with nature.
Take some time to sway in the hammocks under the rustling palm fronds.
But not too much time. Come on, get up, there are still all sorts of mammals, birds, amphibeans and reptiles waiting to enchant you in the jungle.
If you cannot tear yourself away from the lazy life in the hammock, at least keep an eye out for the scarlet macaws. pairs of macaws make a flyover every half hour or so.