The availability of services in San Pedro depends, as you might expect, on where you're staying. If you're downtown, everything should be within a stone's throw. There are banks on nearly every street (including HSBC), dozens of internet cafes and phone shops, and pharmacies dotted throughout the area. All of these can be found on the main square, or in the streets just south.
One of the toughest things to find downtown is laundry service. The closest one is “Almich” on Calle 5, between Ave 9 and Ave 10, southwest of the square.
The downtown post office is located at the corner of Calle 9a SE and Ave 3a SO.
If you're staying at Tamarindo things are a little bit more spread out. Leave the hostel and turn left, then take the first right. After a few blocks there will be a small pharmacy and an ATM on your left, and an internet cafe in the strip mall on your right.