The Caribbean side is definitely calmer, as it is a low profile coast like the east coast of the US. The Pacific side is more scenic, as a high profile coast, like the west coast of North America, but consequently rougher. On the Caribbean, look for beaches running from Cahuita toward the border with Panama. Cahuita can get very crowded so you might want to go further down toward Panama, which I've not done.
The Pacific side is more complex in terms of possibilities. If you are looking for an international standard resort type situation, your best bet is probably up to the northwest, in Guanacaste Province. Elsewhere, you will find more modest offerings. The previously mentioned Manuel Antonio National Park is a beautiful tropical place near Quepos, on the mainland of Costa Rica. But if you go across the Gulf of Nicoya to the Nicoya Peninsula, you will find a lot of more remote beaches on the west coast. I've never been over there but have heard that it is beautiful. If you do go over there, you can get away from the beach for relatively short day trips to the traditional small towns in the interior of the peninsula, meet some of the Chorotega indigenous group who make some very nice ceramics, and possibly even catch a Costa Rican rodeo, which can be a hoot!
In any event, have fun. Just keep saying "Pura vida!" and all the Ticos will smile.