Puntarenas is a a long spit extending into the Golfo de Nicoya. That means there's a beach on one side and an estuary on the other. The beach is rather pretty - it's long and lots of Ticos come to enjoy the small surf and the big sun. The estuary is a long working dock with fishboats and the assorted things that go along with that.
The town is a typical Central American small city, with its mercado, and close shops selling everything from fridges to pirated DVDs. Puntarenas is trying to be a tourist town, so there are lots of kind of pricey restaurants and small hotels and cabinas. It's not that charming, but it is a gateway to the nearby islands and the Nicoya Peninsula.
We enjoyed out time here, but that was mostly because we got a lot of work done and we knew that we couldn't do it anywhere else. it's a good stop for boaters who need parts or work, but otherwise I'd stick to the islands.