Surfing in Dominical is famous (and the Puntarenas region also has Playa Hermosa, not for beginners, and other famous spots); and just a bit farther south (think Uvita) are bath-water warm beaches that are long and practically deserted (and much calmer).
May to November is the rainy season; you might just have scattered afternoon showers. It could also be a deluge. Our experience (Dominical) in the green season was beautiful weather with occasional showers; except for an entire day "lost" to pounding rain (one of the best days of the vacation, as we sat on our deck practically up in the trees at Shelter from the Storm).
Northern Guanacaste will be drier, and there is certainly surfing (water is a tad colder, but ok). Playas Hermosa (in Guanacaste, not Puntarenas), Conchal (ok snorkeling), and Flamingo are beautiful for swimming; Playa Avellana is supposed to be gorgeous and terrific for surfing (1/2 hour? gravel road from Tamarindo). Many like Tamarindo, but we've found it crowded and windy.

Thanks Sylvia! Where did you stay in Dominical?

On our first visit, we stayed at Shelter from the Storm (in Villa Cuesta, which we loved); we were there in the slower green season, and Donna and Daryl gave us a bit of a deal. http://www.shelter-from-the-storm.net/index.html
A bit farther south, past Uvita, we stayed at Osa Mountain Lodge; very nice, new, very reasonable, but 20 minutes up a pretty rough (4WD) road. LOTS to do up there (I don't know how good the road would be in rainy season, though): http://www.osamountainresort.com/

Hola, if you are a very active family I recommend you 2 nights in Arenal activities to enjoy sky trek and tram, Rio Toro Rafting and Rappelling then continue to Manuel Antonio, activities to enjoy outriggers, Campesinos Reserve, kayaks, hiking in Manuel Antonio National park . From here you can flight to San Jose. Enjoy your trip!!! Erik

Sylvia, I appreciate your recommendations.

Erik, we are coming from Nicaragua and northern Costa Rica and will have kayaked and visited hot springs on Ometempe, surfed at San Juan del Sur and Zip lined and hiked at Rincon de la Viejo. Not sure how Arenal will differ. I keep going back and forth if we should skip that area or not.

Go ahead and skip it. It's touristy and overpriced.

I just found this forum and i thought i should make some recommendations, since we are located in the south pacific and this could help you some how.
If you plan on visiting Dominical in June which i think is still a good month to do it, you can look into different stay options, this will depend on your budget.. off course.. if you are more into privacy and prefer to have a place for your selves apart from other tourist, i highly recommend you to consider a rental villa, there are many option around dominical and if you have your own car it will be very easy to get around!

Now, if you prefer to stay in hotels, there are also a few options to consider, here are couple web sites to check: www.villasriomar.com, www.haciendabaru.com

I think this two places are very convenient for two factor, great locations and reasonably priced!

Lastly, from Domical there many cool thing to do, from relaxing at the beach, zip line tours, dolphin watching, day excursion to Corocvado National Park, and even drive 35 minutes up the mountain to a place called San Isidro just to get around and see how Costa Rican people leave, they have a great farmers market on Thursdays where lots of farmers from the region go to sell their products, it is very nice thing to see.

I think you can't go wrong by visiting dominical
Carlos

We love the Arenal area - it has so much to offer. The Lost Iguana is a great place to stay on the edge of town; it doesn't feel touristy at all - great grounds, a huge property.