Dominical is a surf town, you will have a blast. Just be careful of a few things. One is that the ocean on the Pacific Costa has some strong undertows. If you ever found yourself being drawn out to sea, don't fight it, swim Parallel to the beach and try swimming in either further up or further down the beach. In Dominical the beach is pretty shallow for a ways out so you won't have a problem unless you get out over your head. There are also lifeguards stationed at that beach, one of the few beached in Costa Rica to have them. They are there since this is a popular surfing beach and surfers sometimes get in trouble, hit by a board, etc and also because there is a known undertow in Dominical. Watch the size of the waves, sometimes they are fine, other times they are gnarly.
The second concern is just to keep an eye on your stuff. Don't leave anything expensive at the beach while you go swimming, don't leave stuff in a car when not there, etc. Lock your room, lock the windows, etc since this is a tourist town and there are some people trying to live the surf bum life and not have to work and crime is easy on unsuspecting tourists. Dominical is loaded with kids.
Here are the high points you want to do while in Dominical. Go to Hacienda Baru (there ar tours from nearly every hotel) and do their zip line. It is only about 10 minutes from Dominical and costs about $35.
Eat lunch at Maracatu (mostly vegetarian) restaurant and go there for ladies night (I think it is Tuesday. They have live music there.
Check out San Clemente (tons of broken surfboards, a shrine to Elvis, a hippy bus, cool place).
Go to dinner at the hotel Domilocos at the restaurant "Con Fusion" mostly pastas and pretty cheap but great food.
Go to the dance club "Roca Verde" on Saturday night (on the highway just up the road from the main entrance to town and about a $3 cab ride. Great place, good dancing, packed.
Go to Paz Azul Waterfalls to swim one day, ask someone how to get there. You can also go to Don Lulu's Nauyaca waterfalls if you are bored and want to do something one day, ask about tours from any local hotel, huge waterfalls that you can swim in, huge, did I say huge, but you have to ride horses for 2 hours each way and the cost is maybe $45 per person including lunch.
In terms of warnings, be careful of Tico guys that want to take you to their house (I'm thinking like you are my daughter), and wear bug spray at dusk (prime time for bugs) or stay indoors around then. If you are taking the bus from San Jose, again just watch your stuff while you are moving around. The same as you would in any big city.
Also make a color copy of your passport page that has your picture and other information on it. People like to steal passports in foreign countries, keep the copy with you and leave the real one hidden in your house but don't forget it when you leave. Stay together, Be smart....