I hate to chime in where others have already shouted but even though you say you like the beach, keep in mind that this October and it will probably be raining (very heavily) every afternoon.
I did three days in Jaco in October a few years ago and admittedly had a good time, got a little sun early in the morning and partied at the Beatle Bar in the late evening. But in between it was a real pain in the ass to get anywhere because of the torrential downpours. I was in a hotel right on the water (Hotel Mango Mar - nice people run it, chica friendly and like I said right on the water and very near the center of town) and took a nap around noon. I woke up around 3 pm and was starving only to find that the little road leading back to the hotel from the main drag was now a running river about 2 to 3 feet deep. It wasn't until 6 pm that the water receded enough to venture outside again.
Now don't get me wrong, it's gonna rain in San Jose too. But unlike Jaco, there are a lot of indoor activities to be found in the daytime
I wish you luck and a great time on your first trip to Costa Rica