I just returned from a spring break trip to Tamarindo with five of my girlfriends, and I stayed at El Diria. The grounds were gorgeous, the beach view and sunsets were exquisite, and the staff was always friendly. The rooms are fairly basic---no Heavenly Bed or luxurious bath products. But they are good enough, since you are out of the room most of the day anyway, and each room has a large balcony facing the ocean. Even in a room with two double beds, I highly recommend only two people in a room, since the bathroom is pretty small and there is only a small amount of storage space in the room for clothes, etc. It was nice to have a small refrigerator in the room to store lots of bottled water. It is hot there during the dry season--it was about 95 everyday.
There is no gym, but you are right there on the beach if you want to go for walks/runs. And the hotel can arrange for yoga instructors to come and teach on the grounds of the resorts early in the morning for a private lesson.
The buffet breakfast each morning included omelets, fruit, three types of fruit juices, several types of breads and patries, cereals, sausage, beans and rice, etc. The coffee service can be slow, so feel free to get up and get your own. Also, the tap water is potable, so you don't have to worry about getting sick, like in Mexico.