While most people are visiting the much grander (and busier and more expensive and harder to get to) Nauyaca Waterfalls near Dominical, I'll gladly take this more off the beaten track option. The good news is that it runs even in the dry season. I was there at the end of March (end of dry season) and it was great. And it's normally not very crowded unless, like I did, you arrive right before some small tour buses. The cup-half-full aspect of this is that you get to be entertained by all the tourists going down the water slide while you relax in the pool below.
To get there, turn inland off of the Costenera Highway at the BCR (near the popular Sibu Cafe) in Uvita and go about a kilometer. After the pavement ends, you keep going past the cemetery until you come to a large brown sign welcoming you to the Uvita Mountain Community where you'll turn left up the hill. About 300 meters on the right is a very small sign at the entrance to the parking lot for the Cafeteria de Morpho Azul and Waterfall Entrance. See the map below for the Catarata Uvita y Jardín de Mariposas. They're open from 7 am - 5:30 pm.
Turn inland (right in this photo) where that black car just came out of.
Turn left here (see the big brown sign?)
This is it! Turn right down into the parking lot.
The waterfall itself might not be too big, but a visit to this location ranks big on my list
Park in the dirt lot and pay the 2,000 CRC (or $4 USD) entry fee at the window by the restaurant. There are bathrooms at the restaurant you can use before you go to the falls. 
As usual, you'll be better off paying in colones than US dollars. They still use a 500/1 conversion in many place when (at the moment) it's 600/1.
After taking blurry photos of Blue Morphos in flight (they're brown when they land and have their wings closed, so good luck with this one), head back down the path to the swimming holes and waterfall. Keep and eye out for monkeys and toucans! I heard the call of a toucan not far from the parking lot when I was there but the trees were too dense for me to spot it.
Entrance to the butterfly enclosure.
Blue Morphos up close - lots of them!
The trail is nice and flat and easy. You just have to maneuver over some rocks near the end.
Coming up to the first big pool.
Jumping into the first pool.
Some people jump off of the rocks on the far right side (looking up at the waterfall), but that's not really recommended unless you know what you're doing or are with a guide.
The tour group all lined up above the waterfall to slide down.
The Cafeteria Blue Morpho. A pretty setting to grab a snack when you're done.
One of the many small refreshing pools where you can have a picnic.
