Okey doke.
So I assume that the turtle thing is in Manuel Antonio. Near there is the cool little town of Quepos. Nice little place to stay and the beach is great and the sunsets are stupid good. The beaches at Manuel Antonio are gorgeous and they have some scuba diving....Though, to be fair, the coral sort of blows. South of the park there are several surfer towns that all have a ton of beach style hostels and hotels and surf schools.
Puerto Viejo is gorgeous for swimming and there is lots of junlge nearby. You can rent a scooter there and look around. There are tons of places nearby for zip lining and jungle expeditions.
Near Arenal again they have great hiking...the volcano of course....Some very good river canoing....and I think there was some white water rafting, though I didn't do it. I'll look that up.
The is nothing to do in San Jose that you couldn't do in any other city. It's right in the centre of the country so CAN be used as a base, but it's sort of rundown and unfun....Every time I write this some girl comes on and says stuff like
"It's off the chain in San Jose if you ain't a nerd." So believe who you will.
To get you started.