My favorite spot was Monteverde, which means literally green mountain.
There was (no exaggeration!!!) a 30-mile dirt road into the middle of nowhere that you would never have expected to find anything of value at the end of, but, at the end was basically a little town. We stayed in a very nice hotel (one of several) and the dirt road main street had 4-wheel drive vehicles parked all along it on both sides. The cloud forest was all around us and the birding was tremendous!
My favorite sight in Costa Rica was a Resplendent Quetzal. It is a lovely emerald green bird with a very long tail.
It was very entertaining to watch the male try to get in and out of its nest, an old woodpecker hole.
The one main thing that struck me about the entire place was the amazing biodiversity. Everywhere you look there was a new flower and a new fruit that you had never seen. The favorite food of the Quetzal, for example, seemed to be the "aguacatillo" which is a miniature avacado.
My other favorite place there is called Golfito, way down in the south.
(Banana Bay in Golfito)
A couple of driving stories might give you a (not so pleasant) feel for the place. On the road to Golfito, there was a sign in the middle of my lane of the road in the distance. As I stopped for it, it read "ceda el paso" which is basically "yield the right of way." As it turns out, my lane of the road had sloughed off into the river (many years ago???) and I had to yield to oncoming traffic for about 300 yards on the one lane that remained. Also, I was stopped to make a left turn from a two lane road into a market, and, when it was clear ahead, I began to turn. I stopped short when I heard a horn, and it was a good thing; the car behind the car behind me had decided not to stop for us and went careening past at about 30 mph, horn blaring. Luckily, I stopped quickly enough and there was enough of a shoulder on the other side of the road for him to pass and not kill me.
I think I'll end my friends experience with just a few of the pictures of the birds he loves.
