From the airport in San Jose, you can take a taxi (available in front of the airport, once you leave baggage claim) to the bus depot in San Jose. From there, you can take a bus to Puntarenas. The trip will take a couple hours depending on traffic and weather conditions, but its inexpensive.
Local buses leave from downtown and are relatively inexpensive. For travel to places outside of Puntarenas (ie, Jaco, Manuel Antonio, etc) there's a main bus station. Buses are usually on time and sometimes even leave early, so plan accordingly. Don't be surprised to see vendors on buses selling snacks and drinks.
Taxis are easily accessible and can be flagged down in the street, or you can call ahead to schedule a pick up.
Most locals ride bikes or walk. You can rent bikes from any of the bike shops at really good rates. Just make sure you keep your bike locked. Helmets may be hard to find though, so you might want to consider bringing your own. Remember that pedestrians do not have right of way and that you must ride your bike in the street (not the sidewalk), so be careful!
Walk along the Paseo de los Turistas and at the end of the Paseo, eat a "Churchill". This is as traditional Costa Rica as you can get.
Walk out onto the main pier (when there is no cruise ship docked) at night. Many locals come here to drink, fish, and hang out. A wonderful place to view stars and the small lights of distant cities.
Visit the new public park on the end of the peninsula. Walk along the south beach until you see the large rocks that make up the sea wall, there are walkways with lights and benches on top of them. A public basketball court, playground, and many benches are a wonderful and quiet place to watch the sunset from.
Take a ride on one of the inflatable banana rafts being towed by boats on the south beach. On busier beach days (holidays, cruise days, and weekends), men will offer rides on these tubes for about 1000 colones (2 dollars). Life jackets provided.
-Apretados- A delicious ice cream like snack that comes in a variety of different flavors. You won't find these sold in any store, but out of the front of resident's houses, and will run you about 250 colones (50 cents). Holding the plastic bag in your hand, tear off a small corner with your teeth and suck out the melting apretado.
-Granizados dos Leches- This sweet, cold snack is similar to a snow-cone, but with a little something extra. Wait on the beach for a cart pushing vendor, and order a Granizado dos Leches in Kola (fruit punch), Chicle (bubblegum), or Naranja (orange) flavor. Shaved ice with powdered and condensed milk, served with a spoon and straw. Costs about 800-1000 colones (2 dollars).